I'm setting up a single speed 29er and Jenson has a set of 180mm XT 10sp cranks on sale - I'm a large rider and would like a tad longer crank. Can I use the existing chainrings w/the surly rear cog I have or do I need a 9speed specific chainring? I assume they're still a 104 bolt pattern - will it really make that much of a difference on a single speed? How about bolting on an aftermarket ring like a blackspire or e13 (which state they are 8,9,10 sp compatible)?

In most cases you only need to match the bolt circle (BCD) of the chainring and crank arm. The bolts themselves are standardized so that's rarely an issue.

However, on cranks where the chainring also rests on a shoulder as do most outer and some inner (or middle) rings, there can often be a bad fit when mixing brands. This can always be resolved with a hand file except for the cosmetics (unsightly? gap) on the outer position.

I find that a good fit between the ring and the crank's shoulder improves concentricity and seems to prevent loosening, so I'm careful to confirm that detail when mixing brands. Good shoulder support is especially valuable on SS cranks, because there's more back and forth torque on the chainring, and because concentricity is more critical.

Cool, thanks for the answer - I didnt think about that part of it.

AFAIK 8/9/10spd as it applies to cranks only refers to the thickness of the spider between the rings.

Shouldn't have any relevance to SS beyond a mm of chainline...