I've killed around 50 with a 3.5" 2 oz 12 gauge load. I've mainly used 8s, but used 7s in NE because of a crazy law they have that made 8s illegal. I've found that the larger pellets will make your gun shoot a little tighter patterns. Some see that as a good thing, but my patterns with 8s are tight enough and I considered the 7s to be a disadvantage for me. The 9s would work fine if you wanna keep using them.

Center the pattern on him and the shot size doesn't really matter with TSS. I think there is probably some distance at which 8s would be better than 9s in a 12 gauge load. The 9s will run out of penetration before the 8s, and theoretically there should be some distance at which 8s still have enough penetration and also enough pattern density that it should still make reliable kills. However, that distance would be so far that it isn't worth talking about in actual turkey hunting.

I think it's a question that might be important if we are talking about maximum range to shoot a coyote. With turkeys, it shouldn't make any difference. Use whatever size you have, but be aware that the larger shot will produce a tighter pattern and you might want to use a different choke. Good luck with it.