Sentinel: Tech Armor or Barrier in ME2/3?

You can use both at the same time in both ME2 and ME3.

Not really sure why you would use both in ME2 (just Tech Armor is generally preferred), but go for it. Geth Shield Boost may be better early instead of Barrier since you can get the Tech Cooldown upgrade sooner than the Biotic Cooldown upgrade.

Fortification is better than Barrier in ME3, since it provides better damage protection bonuses. It can also provide better power damage bonuses too, depending on evolutions taken.

I love Fortification in ME3. It's my most frequently selected bonus power. I have used it on Sentinels, but it's probably a bit overkill. You will likely get more out of AP Ammo, Stasis, or whatnot.