20,000 Human colonists

Because of how much it would cost. And how no one that isn't crazy would make that kind of investment. 

It makes perfect sense for the new kid on the block to make waves. Everyone else has been following the path the Reapers laid down. Humanity developed along it's own path and came to the table with a fresh approach and different eyes.  It's not surprising they saw new ways of doing things that the others hadn't thought of.

I don't see your argument at all.

All these windfalls that "just happen" to a singular character (or species in this case) is still very much the calling card of a Mary Sue though. It's not enough that humanity is making waves during their very, very brief time as a part of galactic society, but that every major advance, every innovative leap forward, every new development of ideas comes from humanity. Not once do we see these other species, all of whom have far more infrastructure and experience involving interstellar life than we do, make any sort of major contribution to society as a whole like humanity does throughout the course of the trilogy. Sure Shepard and humanity's arc is about dealing with the Reaper threat, but do we also have to be the ones who:

Think of the concept for carriers? Who conceive of a universal, cross species medical treatment? Who are the only ones that can track down a solid lead on the Shadow Broker? Who can (potentially) hack the entirety of the Geth Consciences (and all other synthetics in the galaxy) with a single autistic mind? Who are the ones to discover both the Rachni and Thorian and be in the lead when conducting studies of them? Who are the only species in the entire galaxy with enough 'genetic diversity' to satisfy the Reapers' criteria for making a new Reaper and have the potential to be better at everything than everyone else because of said genetics? The first species to create a communication device outside of the Mass Relay network (QECs)? The only species able to piece together the mystery of the Leviathans and bring them into the war against the Reapers?

Why is it that we are economically out pacing the Volus, the very species who created the rules for the galactic economy and have been running it for the past two thousand years? Why is it that we are better at information gathering and covert operations (with a single terrorist organization in most cases) than the entirety of the Salarian Union, who have been specializing in that very thing for over two thousand years? Etc. Etc.




As for the whole Take Back Earth in ME 3, why would the galaxy care that the Reapers are focusing there prior to the Citadel being moved into low Earth orbit? The major advantage of the Reapers that we are told time and time again is that they don't have home worlds or supply lines to worry about, so even if the forces of the galaxy stopped the completion of the human-based Reaper there are still a gazillion of the the things flying around the galaxy so what's one more Reaper going to do? All the galaxy would have done is waste valuable ships and resources on the liberation of a world with zero tactical significance to the war effort. At least a planet like Palaven was still being actively contested. If anything, Admiral Hackett should have been gathering his remaining forces there, to help hold the front line. Sorry Anderson, London is great and all (I guess), but putting all of the galaxy's conventional eggs in one basket to save it is beyond foolish.  

What's more, when the Citadel is captured and moved to Earth; and all of the galaxy has an actual reason to go there; and the allied forces win the day we don't see the symbol of galactic unity and co-operation put back in it's original place as an honor for all the species who fought and died to save the galaxy. No, it's left, like some trophy, over Earth (in all three endings to boot) practically screaming that the human home world is now the undisputed head of galactic government and policy. 

I could be wrong but my understanding was that the Reapers tried to lead technology down a particular path by leaving behind the Citadel and the Relays for the races to reverse engineer.  Since humanity only discovered the Relays and the other races very recently their technology had been free to develop differently.

Either way, they bring a fresh perspective and that can often lead to new ideas and new innovations. It's not that they are necessarily better than the other races just the fact that they look at things differently can open up new ways of doing things.

That's the exact opposite of the Reaper plan.

They left the Citadel and the relays behind so the races would become complacent.  The relays are to keep the races from looking further.  They already have free transportation to any number of worlds, and a hub where it can all be directed.  Why think outside this nice, comfortable box?

Sorry, you are mistaken. While yes the Reapers left the Citadel and Mass Relays for the cycles to follow a particular path thus make the harvests more efficient, humanity had not developed along a different path. Humanity essentially hit a brick wall when it came to interstellar travel until they found the Prothean ruins buried on Mars, which seemed to hold all the answers and left humanity to the discovery of mass effect technology and the Mass Relay inside Charon. That Mass Relay led to others which led to the Citadel. This happened with all the other species of our cycle because like the Citadel and Mass Relays the Reapers left certain Prothean ruins behind to help guide new races to the same kind of path the Protheans and all previous cycles used, with the added benefit of keeping the Reapers hidden from them and instead have the galaxy think these things were from the previous cycle, not a trap. With those 'gifts', humanity like all the other races in our cycles and all previous cycles developed along the same path and perspective since there was no need for them to look for another way.

An anthropologist suggested that 150 people on a colony ship that didn't interbreed along the way could sustain genetic diversity.  Another guy said 10,000 but that 40,000 would be better (to account for potential deaths in transit).

www.popularmechanics.com/space/deep-space/a10369/how-many-people-does-it-take-to-colonize-another-star-system-16654747/

Most of the colonists should have been Scandinavians.

See this is why they should use tank-breeding to bolster their populations.

Or possibly drop some folks onto planets in ACUs and have them mass produce millions of bots to overwhelm any defenders.

Seems reasonable if they did a medical check of each volunteer in order to see if they were fit for repopulating.

Yep, definately not a Bioware thing. So regarding procreation there will be no limits on colonists.

There doesn't need to be more than 20k on the Arks since there could be 5-10k on the Nexus itself.

Plus, if they brought over egg and sperm samples, they could increase the population and genetic diversity even further. Granted, convincing people to become inseminated with random people's DNA who're billions of miles away and 600 years in the past might be hard, but I'm sure some would bite.