Tiling questions

It's now warm enough to begin tiling my kitchen floor. I have two topics I'd appreciate your thoughts on. (1) Expansion joints and (2) mortar & grout. The area to be tiled is a little over 30 feet in one direction and 22 feet in the other. Part of the substrate is 1/2 plywood over 5/8 T&G plywood, the other is 1/2 Durock over 5/8 T&G plywood. The support structure is 2x10s on 16" centers with no unsupported span of more than 12 feet.

The tile is Daltile, 8 x 8 x 5/16.

I'm not thrilled about putting tile on plywood so I am also covering the plywood in those areas that don't have Durock with an isolation membrane.

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In looking a various sites I find a wide range of recommendations recommendations for interior floor expansion joint spacing. The Tile Council of American says no less than every 30 feet, however they caveat that by saying that in areas where direct sunlight will occur (and it will here) spacing them as close as 12 feet is appropriate. The expansion joint that I'm using is Schluter's DILEX-BWS

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Any thoughts on expansion joint spacing? Obviously for aesthetic reasons fewer is better, but the appearance of a cracked floor trumps all.

I'm in a remote area so supplies are difficult to get. Home Depot is a couple of hour drive but I can get their Custom mortar there. They have several grades of mortar. So far I've used their VersaBond mortar to bond Durock to plywood and plan to use it to put the isolation membrane down also. My question is what to use to bond tile. Can I continue to use VersaBond or should I step up to their FlexBond mortar?

Also, grout is a big issue. I'll be using a very dark, almost black grout and although I don't expect the floor to be wet I am concerned about efflorescence and what to use for grout. Is Custom brand acceptable?

Thanks,