Yes you can mix and match 6 and 8 ohm speakers. However, this sounds like you want to mix and match different makes, which isn't usually recommended. The reason for this is that any one designer designs speakers with a particular philosophy of design that may differ to the next designer's philosophy, thus coming out with not only different impedance characteristics, but a different 'voice'. Therefore B&W speakers sound like B&Ws whereas KEFs sound like KEFs. Placing a B&W centre speaker between two KEFs isn't recommended because the B&W voice differs significantly to that of the KEFs. This makes the centre stand out and therefore the soundstage sounds artificial. Therefore, it is better to match the make in a surround sound situation, even if the match is an inferior one. So for example, if you had KEF Reference main speakers, but only enough cash for either a KEF Coda Centre or a B&W CC6 centre, you should choose the KEF even though it's not - on its own - as good as the B&W centre.
Now, if you are matching makes and it's just a question of different impedance, the main difference you will experience is that you would have to setup the receiverslightly differently than if they were the same impedance. In general, this won't affect the receiver significantly.