Most AVRs won't give you option of impedance for individual speakers
In order of preference:
1 - All speakers from the same family - all will typically have the same impedance curve
2 - LCR speakers from safe family, preferable at higher nominal impedance , you can get away with surrounds/wides/atmos from different family and with lower nominal impedance as your driven channels don;t have to spend too much grunt driving these
Unless you are using a separate amp , it is better to keep at least the L-C-R at nominal 8 ohms because the amps in almost all AVRs aren't particularly beefy. 80-90% of the power draw happens with these channels
Couple of other things to keep in mind if using lower impedance speakers on any channel and esp if on all channels
Use a powered sub and set a high enough cross-over - 80 hz or 100+ if you want to play it safe
If using low imp. speakers on non L-C-R, at the very least set crossover very high for them, even 120 or 140
Most amps do not have problem driving low imp loads for higher frequencies but you can expect channels set at a low cross over point to get overloaded with low imp speakers during passages with lot of bass