RE: Sites for gas setup advice.
Not familiar with the batshare wiring - I'm not a fan. You'll need one manual on/off switch for each battery pack as a rule, but again, that may change with the batshare?
You could use an Ultra IBEC, but it's big feature is voltage regulation (for 6v. ign modules), and you don't need that? The 20.00 Rcexl switch will work fine for you.
The servos will be fine.
Regarding flight time, I think it's safe to say you could do a LOT of flying on a single charge, but exactly how much is going to introduce too many variables. The only real way to tell for sure is by noting what it takes to recharge your batteries after a flying session? My bet is you could charge your batteries every other time you go flying? -Al