Q: Is placing smaller PA speakers behind the main speakers facing the band a better solution than using floor monitors?

A: No. Properly placed floor monitors (at the base of the mic stand) project sound at the null region of directional (cardioid) vocal mics, which increases gain before feedback. If you tried to forego the floor monitors and depend solely on sidefills, you would probably wind up with feedback while trying to get the mix loud enough to satisfy all of the players.