On the question of mounting a 10"-13" on a 6" Eq. mount, don't. Won't work. ESPECIALLY for Photography!
On the question of opting for a Equatorially mounted scope for "the occasional photo session", don't. You're asking for a lot of extra weight, complexity, setup time and fuss, not to mention expense, for a design you'll take advantage of only part of the time. Whenever possible, keep it simple. I've seen many folks get into this, thinking that they want to do photography, and wind up with a scope that's such a pain to use, that it just sits. Unless you want to hit the photography thing real hard, don't! :-)
I tell you these things as an astro-friend!
see-ya!
Mb (Mike Bagneski)
P. S. I can't imagine how this COULD be offtopic! :-)