I think these are the usual problems you get with bands, creative individuals in the same room will cause one or another to try a bit of oneupmanship. It is an unworkable rule to impose, so just stand your ground and make sure you get your points across.
This sort of thing caused me to leave my first band in the 90's, we recently got back together and some of it is resurfacing but is usually cut off before it can become a problem, were a lot older and wiser.