Only buy the guns you want your allies to use. Sell off everything else and they will be forced to use it. That way you can create a specialty for each character.
If anyone is wondering whether to pull the trigger and spend the $5 to get the full game, do it! The depth of gameplay coupled with an actual storyline and RPG elements make it one of the best TD games I have ever played.
Just use the dumpster once you get it and at the end of every combo dumpster dive and tunnel grind. Gets me about 25k-30k each time! + so people see it!!!!