Thanks for your interest in the rules.
It's possible to play with multi-based figures but it has not been tested much as the rules were not intended for it. I know that some people have been playing with rectangular bases with three figures each, and it worked. If you use bases with 1, 2, and 3 figures, you can probably keep count of casualties by replacing a 3-figures base by a 2-figures base when one of its character is killed, etc.
You mention that you have recently downloaded the rules so I suppose that most of your players are not familiar with them. I would suggest that you run some small games before organizing a large multi-player event. Some players may be uneasy with the RPG idea; other may have difficulty with the simultaneous moves; if one (or more) of the players hesitates all the time before moving while others players are making their moves, it will not only slow the game but you can lose understanding of what everyone is doing and it can ruin the game.
Feel free to ask about any question or problem.