Jeremy, I think unless you're talking about super expensive alnicos like JBLs, that ceramic will tend to give you the bigger, deeper cleaner tone and the alnico the more easily overdriven and compressable tone. In my amps where I've combined the two types it's the ceramic that adds the cleaner tones and the alnico that provides the more easily overdriven tones. So I'd recommend alnico for the 10's and ceramic for the 12.
You could come up with an 8 ohm load by going with two 8 ohm 10's in series (= 16 ohms) and then connect that combination in parallel to a 16 ohm 12. That'd be two 16 ohm loads in parallel and would give you 8 ohms total.