The tickets are 1200 roubles online or 600 roubles in person. Okay, it's not much - but why pay extra when you don't need to?
If you get a licensed tour guide, they'll provide the ticket.
Don't find someone milling around. If you go on an individual tour, organize it beforehand. If you want a group tour, go on an official one organized by the Hermitage.
Russia doesn't really have street food and most Russians don't eat at a roadside kebab stalls (which do exist, but not in any place tourists commonly go. They are mostly found in poorer suburbs).
Termok is the Russian equivalent of McDonalds. Interesting to visit and try out, and some of their blinis are quite good - I really like the farm one. Kroshka-Kartoshka is pretty poor IMO. Both have English menus, so you can just point at what you want.
SOmewhere else to go to see 'the real Russia' is Udelnaya market. It's a large flea market in the north of the city next to the metro. It's pretty interesting.