This may be self-explanatory, but when playing solo, can I just make a squad of 4 soldiers anyway? Likewise, 2 people can each control 2 soldiers? (this is assuming 4 is the max squad size?)
Definitely! As long as you are taking the Soldier turns in the same order each time, you are good. Nothing stopping solo play with more Soldiers (you'll just likely want to bump the Difficulty up to match the higher squad total). And yea, 4 Soldiers is the intended maximum.