As the other replies suggest you'll have no difficulty with 4/6/8 ohm speakers except in one particular instance.
The receiver set up allows for two positions of surround speaker, A and B. These can be used independently (e.g. A for film and B for surround sound music) or together (A+B for both, which, incidentally, is the configuration I use). If this latter option is chosen one must use a minimum 8 ohm speaker or damage to the amp could ensue.
With two 8 ohm speakers in parallel the amplifier is effectively "seeing" 4 ohms, so anything lower than 8 ohm would begin to stress it; i.e. a 6 and 4 ohm speaker in parallel is effectively a nominal 2.4 ohm load. Definitely a mix to avoid.