I posted this before a while ago but for some reason it won't appear on my posting history.
I am on my 3rd playthrough of this game with my 3rd character who is an Engineer. I already used Garrus and Wrex and got the Ally achievements for both on my previous playthrough.
Out of the remaining 4 squad members who are the best for an Engineer playing on easiest difficulty (to get tech ability achievements and 2 ally achievements) and who are the best for an Adept (for my next playthrough using my old character to complete on Insane, achieve level 60 and remaining two ally achievements)?
It should be possible to get the ally achievement for both Kaidan and Ashley because I just need to do all missions and side missions before I do Virmire. (Check this please).
In any class you get, you basically want to pick the squad members that balance out your entire team. As an engineer, you're left with Ashley (soldier) and Liara (biotics). Getting Tali or Kaiden would be pointless since you're an engineer yourself.
As for Adept, the ideal choice would be to pick out another soldier (or someone with strong armor: Wrex, Garrus, Ashley) and then either Tali or Kaiden as your engineer.
You'll probably have to do another playthrough to get everyone unless you're doing this in easy mode. Then you can probably get by with what you have (but I'm unsure if you can get the ally achievements on easy).
You don't "have" to balance your team out, it just works better. It just depends on the type of runs you want to do.
Soldier/No boxes run: Shepard/Ash/Wrex
Adept/No boxes run: Shepard/Liara/Wrex *I'm sure we all know I've always HATED Kaiden by now.*
Full tech run: Shepard/Tali/Garrus
There's a few other little combos you can do, and if you want to switch a party member out for someone else of the same run style, that's fine too. All depends on how many playthroughs you want to do and how much you want to mix things up. Maybe sometime I'll do the.........
Useless party run: Sentinel Shepard/Ash/Kaiden
Just for laughs and maybe a headache. --- "I don't see how a clothed female is more attractive than a naked one." -Wrex
I usually do try to balance out my teams as it's very useful to do so. What I like to do though is be a pure class so I can pick other "pure" classes for their extra abilities. For instance I'm a soldier with singularity right now and I bring along Liara because two people singularity means my enemies are ****ed. I also bring along Tali because she's badass for one and two she has AI Hacking which can, IMO, be one of the funniest abilities to see in action.
By no means do you need to follow that though. I just prefer to have a little something of everything in my squad as when I don't I find out it would have been useful if I did.
Well I've decided now to use Liara and Kaidan for my Engineer character on normal difficulty (to speed things up) and for my Insanity playthrough (I'm going to need a combat specialist no doubt there) as my Adept I'm going to use Ashley and Tali