In my current playthrough, I've got robin's aliens (waspite and gazer), the MiB (with Solar's expanded armory for MiB), Solar's extra UFOs (with Solar's expanded armory), Luke's extra UFOs, Shoes' Soldier Diaries, and civilians' Terrain addons. So far, everything has been going very well.
A big game changer that isn't a mod is using UFO Extender's accuracy style. It makes auto shot far less useful in long-distance fighting and puts a much bigger emphasis on positing your soldiers for sniping, which goes along very well with weapons like Solar's plasma sniper.