If you use Cardioid mics, position the instruments so that the 'off-axis' of the mics will point at the instrument you DON'T want to pick up ie actually point the amp at the drums. This will mean the direct sound is being cancelled out. If you point them AWAY from each other then the direct sound is going straight into the mics.
Baffling is good if you can get it and have the players next to their amps definitely, but the other most important thing is Volume. if you're instruments are equal in volume, then the spill from other instruments should be minimal and manageable. When you have one quiet sound and one loud sound, you'll have to boost the mic amp of the quiet sound to get it up to level, but in doing that you'll boost the spill from the loud sound. So balance your guitar amp to your drums. Don't let your guitarist dictate the level because 'that's how he get's his sound'. It'll sound exactly the same on mic quiet or loud.
You should be OK in a room of that size, maybe used a few rugs (on the floor and walls) and curtains to try and deaden things a bit, but that's not necessarily needed. Check out this band...
ROWSE, at the bottom of the home page there's a link to a player. Have a listen to Stray Dogs or two lonely souls. Drums, bass, two electric guitars, done in a quite reverberant room of similar size to yours, all live, no baffles, by just balancing the levels.
Embrace the spill and be aware you're going to get some and go with it. Make sure you try and get good performance more than anything. If you get this the spill will be almost irrelevant.
MixTips: Performance Is Everything