No optional characters run of the main game would be...
Reimu, Marisa, Remilia, Sakuya, Patchouli, Chen, Cirno, Youmu, Alice,
.
The main difficulty would be that until you actually -beat- The Trio(tm), you don't have a full set of 12 characters. And your tanks are... well, sort of subpar (Remilia's decently survivable, but I found that she was beaten out in almost every way by Meiling for tanking purposes; Patchy is an anti-magic tank at least, Youmu, Alice,
are all semi-decent at tanking, kind of?
I think it'd be possible though. You still have Reimu, which is the biggest one. Hell, I think everyone on that list is someone usable, with the possible exception of
? (I have never found her all that useful)
Alternatively take the Level 1 Everyone save and beat the main game with the Plus Disk cast ONLY.