Maybe the folks on stage are too loud. It is always a good thing to have low level on stage, and this is even more true as the stage is small. Appart from feedback problems, it also helps having a good sound in the FOH (less bleeding). I know this is hard to explain musicians to play at a lower level (there is the groove you know, etc.). Not beeing able to hear oneself is a relative question of monitor too low or stage sound too hot. Technically, lowering the second parameter is more efficient.
My stage is only 8'x10', and I never had any problem of feedback with any voice mic, including SM58, Beta58, Beta87, SM86, Audix OM3, Beta57, KMS105... I even never had to play with the EQ to eliminate feedback on voice mics. However:
- the FOH is made of coaxial speakers, and there is at least 3 feet between the "FoH plane" and any microphones
- monitors are good coax as well
- I have 31 bands + parametric on every outputs (digital board)
- I put 4 inches rigid rockwhool on stage left and rear wall
I have more problems miking the upright piano with Beta98. I cannot raise the volume as loud as I would like sometimes, because this starts feeding back with the FoH (no monitors - do not even think about it);
So, a few tips :
- lower stage volume if possible
- good placement of monitors with respect to microphones, taking directivity into account
- maybe less instruments in the monitors
- check the remainging feedbacks are not due to the FoH
- then, if it does not help, maybe you'll have to invest.