Wow, you don't have a lot of PA to work with.....
Running 4 mons off of one 500w power amp is going to result in underpowered monitor mixes. Not only does that lead to distortion, but it is bad for the speakers. You didn'
t mention any EQ's in your list of gear. How will you control feedback in these monitor mixes? And how will you shape the house mix with no EQ?
As for the mics themselves, I am not familiar with the Samson mics (although, in my experience, Samson = p00p), but I'd say, stick the condensers on the percussion, and use dynamic mics on the guitar amps/vocals. Don't bother with drum overheads as all you will end up with is bleed from the rest of the stage. In a small enough room with a loud drummer, you might just wanna do a kick snare overhead setup and call it a day. Get the overhead right over the kit...not way up in the air.
I like to have bands turn their amps to face themselves instead of the traditional approach that most bands take by aiming their amps at the back of their knees. Small combo amps can be leaned back and placed in front of the guitarist(s), aiming at their ears instead of the back of their legs. Bigger stacks of amps can be turned a full 180 degrees and mic'ed from the backside. Take a DI line from the bass player, don't bother micing his amp. Same goes for keyboards.
It's all about stage volume.....get the band to keep their stage levels down. Aiming their amps at themselves instead of at the crowd will help. If they are all cranked up to 11 on stage, the 500w amp driving 4 monitor mixes is going to be useless.
Soundchecks are nice and all, but a soundcheck isn't going to make or break the show. The sound will be completly different when you get 250 people in the room and the humidity and tempature go up. I usually get a line check and use the first song to dial in the mix. A good engineer should be able to get the mix at least in the ballpark with a simple line check. Only put into the monitors what the band asks for. Adding guitars and bass to the mons is only going to make things worse. Use them for vocals exculsivly unless someone asks for something else. The only exception is for the drummer, who, assuming he is on his own mix, should get all the vocals, his kick, the bass guitar, and maybe some guitar/keys if he needs it. If everyone else has an amp, they should be using that as a monitor, not pumping it through the wedges.
My best suggestion here is to have whomever owns the PA system buy some decent mics. If you put shit in, you get shit out. Buy some Shure Sm57's and 58's, a decent kick drum mic, and a few DI boxes. The 57's will cover just about any instrument you encounter. A pair of condensers are nice to have around too, but I would concider them secondary to decent dynamic mics. Oh, and get some EQ's in line so that you can actually adjust for feedback/standing waves, etc...
Best of luck.