I'm using the Etymotic ER6i (they are basically the same as my earplugs) and having tested them in my also tiny jamming room with full drumset, guitar (2x 60w amps) and PA system (for vocals), i can say they can be heard over all the volume. Not sure how loud the volume was cos I can't adjust the volume on my pocket metronome. But as a guide it was pretty loud, so much so that it kinda hurt after a while.
But seriously, you should be able to hear the other instruments if the volumes are balanced correctly and you have the amps, speakers, etc angled properly. I play the drums and I can still hear my bro's guitar very clearly (he outputs his guitar to 2 amps, one directed towards him, one towards me).