Tips are traditionally given at the end of the cruise. If you want to charge your gratuities to your account (or if you need cash) go to the Purser's desk a few days before the end of your cruise and take care of this. You won't have to wait in as long a line if you go early.
Either way envelopes will be delivered to your stateroom on the second to last night of the cruise, use these to deliver your tips on the last night of the cruise.
When it comes to dining, that table size and assignment is only when you go to dinner in the main dining room. If you head to any other meal in the main dining room you will be seated at another table and may even get a private table if you would prefer. Dining in the alternate restaurants are typically at private tables just like in a shore side restaurant.
BTW should you decide that you would prefer to dine during the early seating, would rather a different table, etc... speak to the Maitre'De as early as possible. If there is any way that they can honor your request they will.