Since I can't select the companions Perks, is there a preferred weapon each companion uses? I noted Piper immediately pulls out a 10mm, so I assume that giving her an upgraded 10mm may be best. I assume Strong and Cait like melee. Etc.

Another question, do they have unlimited ammo?

I do not know I still only have used Dogmeat after playing 10+ hours but then again I walk everywhere and for the most part just exploring and getting lost. I think I can enlist Garvey but Dogmeat would get mad with me and likely poop on the floor of my house and Codsworth would likely burn him and then he would become dogmeat.

If a companion has a default ranged weapon it will have unlimited ammo. If you swap it out and give them another weapon you will need to give them ammo for the new one.

What if I give them the same weapon, just upgraded. Like if I give Piper a upgraded 10mm. Will that one need ammo?

According to the guide, it only works with their own version of the weapon. I'm guessing that this is to prevent your companion from becoming too OP.