That's totally fine! Each ball usually only has one troop inside anyway, so no problem!

There is definitely a time limit - the stat boost lasts, but the instant firing (musketeer, catapult, archers) only happens at the beginning of each battle. If he doesn't use it when he starts fighting, he can't use it. It's also a one-shot per battle thing, so he can't just magically start a firing off stuff whenever he feels like it. He can also only use the troop ball if he's already carrying it at the time of the battle start. So if he picks it up halfway through, sorry Donut, no support fire for you.

Does that seem fair?