If you want to run the band in the PA at those smaller venues, I would suggest cutting all members backline speaker count in half.
Otherwise, just put vocal and a touch of kick(s) in the PA, let mic bleed fill out the rest, and mix it all to the natural level of the drums.
Tjere are advantages to putting more of the band in the PA, even if you're not using it to make them necessarily louder than they already are. They will still soumd "bigger" and better dispersed throughout the audience. If your guitarist likes to crank up that 412, he'll be making the ears bleed of whatever poor souls end up on center with his rig while giving the rest of the crowd more mud.
If you're micing drums in rehearsal, you're already used to playing way too loud to get any kind of quality sound in a 100 person venue (assuming 100 people make the place look full.)
Assuming you can control stage volume to some degree (not quiet, but not blaring either, should be able to hear an un-mic'd kick drum through the noise)....I would do as follows with 8 channels.....
1. Lead vocal
2. Back vocal
3. Back vocal
4. Kick 1
5. Kick 2
6. Single drum overhead
7. Guitar
8. Bass
And make damn sure the vocals are clear and easily understood. A little cut on the vocals eq somewhere in the lower midrange (250hz-ish give or take, depending) usually helps this.