Well, Captain, it looks to me like you already have a fairly well established food business. I'll go out on a limb and say you probably know your clientle.
It looks as if you already have the dough part of the pizza equation worked out.
I thnk the plan to initially have someone else make the dough is a good one. That way if things don't work out, you're not out equipment and supplies.
I would think as a pilot experiment, 3 to 4 types of slices to start with sounds right to me. I am guessing that regular cheese, and a pepperoni, plus 1 or 2 others would seem reasonable as a start. And I think offering over the lunchtime as a test-market way of introducing them sounds good to me. I have to say I am not as familiar with establishments that offer slices as opposed to whole pies of one size or another, so I don't have any knowledge about warming pizza slices or keeping them warm. I trust you have this well worked out in your plan already.
I have a few more questions, if you don't mind.
It sounds to me like your are looking to make a traditionaly, North American, no frills type of pizza as opposed to some gourmet style, is that right?
Given that you already have a diverse menu, what made you think to add pizza to it? Will this perhaps make your offerings too diverse and give your establishment less of an identity to current or future customers?
Have you figured out how you are going to price these slices, given the increasing cost of wheat flour and other ingredients that were traditionally very cheap to purchase?
I'm not trying to discourage you, but these were simply questions that came to my mind as I read your post. I wish you the best of luck.