Bravo !
Nothing can stop Yoga the great
.
There's no need to take a gnome if you don't want to, it's most important that you like the character you're playing, fighter/mage is still powerful and the game is beatable with any class or party.
And I guess twillight will be dancing with joy that he doesn't have to be a crazy little gnome.
A: Do as you wish. I don't know how you like them. If you dismiss them you can take away their gear, but they are quite funny and entertaining to have around and can help in hard battles.
A2: You can revisit most areas in the game almost all the time. You won't be able to return to Candlekeep for a long time however.
A3: No time limit. You can rest for 1000 years now if you wish. If you want to keep Xzar and Montaron you should go to Nahkel with them some time, but you don't have to run now. Most party members want something from you, if it's done (like getting to Nashkel for Xzar and Monty) there's nothing urgent to do any more. There are only a few timed quests in the game and you're told when they start.