I'd actually go with 7.6 unless you can get a *lot* of ram, 8.1 is nice, but 7.6.1 can be really speedy, and System 7's bad 68k emulation code can be replaced (and many apps tremendously sped up) using Speed Doubler 8, which can be had from http://www.system7today.com
7.6.1 also seems to handle low memory availability a little bit better than 8.1, because it's got those few fewer things built in, many of which you might not even notice the absence of.
I also recommend not using virtual memory if you can help it at all, VM slows things down significantly.
Other than that, congrats on the 6100 and the cd drive, especially if that cd300 is the external type, those are really valuable for use with older Macs, since even A/UX supports them. (and A/UX is super picky about cdrom drives)
7.6.1 also seems to handle low memory availability a little bit better than 8.1, because it's got those few fewer things built in, many of which you might not even notice the absence of.
I also recommend not using virtual memory if you can help it at all, VM slows things down significantly.
Other than that, congrats on the 6100 and the cd drive, especially if that cd300 is the external type, those are really valuable for use with older Macs, since even A/UX supports them. (and A/UX is super picky about cdrom drives)