If you post which ship you're planning on sailing, someone should be able to give you a list of the specialty restaurants and what, if any, the additional charges are. We hardly ever eat in the specialty restaurants -- particularly the additional charge ones. On 4 NCL cruises, we've eaten in ONE pay restaurants, and that was mainly to accomodate some friends from a roll-call. Our last cruise on the Pearl was the first time we'd tried any of the non-pay specialty restaurants, so it's very possible to eat all your meals in the main dining room and still enjoy your cruise.
As far as eating with others, you can ask to share a table. Our experience, at dinner time, is that you rarely have anyone else who wants to join you. At breakfast and lunch, the other passengers seem much more willing to table share. My suggestion is that you get involved with the "meet on board" from this site if there is one going, or the roll-calls from one of the other cruise message boards. For us, that's been the easiest way to find other passengers who want to share our dinners. Send me a PM or email if you need further information.