I found that even grossly mismatched speakers worked OK as surrounds as long as the front 3 speakers were matched OK. Didn't work well for 5 channel music (plan on playing most music in 2.0 or 2.1) but was fine for movies.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8