I would suggest to think a little bit more before you start
Sorry if I sounds annoying (again) but I think it is better to have a proper and researched start then starting too fast and make (expensive) mistakes.
First, think about what you exactly want to do. Most of use here tent to go for the historical view, rather than a purely martial arts approach. Both are fine, have their own (dis)advantages and best would be to combine them to some extend. So, basically the first question would be what is more important: martial arts or historical re-enactment.
Also ask yourself the question how experienced you are in martial arts. Using weaponry really asks a lot from you and everyone involved and should only be done with proper training under knowledgeable supervisors.
So, do you go for the martial arts approach, just find a nearby SCA group and see their rules. They probably have enough guidelines of what kit (NOT HISTORICALLY!) you need to be allowed to 'enter the arena'.
Next, again I would suggest you to get some books before anything else. Really the best start, if you would like to take a historical approach, is to read about the subject. Read some more books, and read even more books. Now, when you have read some more, you will start knowing a bit about the gladiatortypes and their equipment and you can answer your 2nd, 3rd and 5th question yourself
About your forth question. I think that sounds like SCA. At least in Europe most groups don't challenge each other for safety reasons, as most fight their own way and with real weaponry.
Ow, finally I would of course would anser the remaining questions:
-to fight you need your own kit, which depents on which kind of gladiator you would like to portray, same goes for your opponent.
-to fight you need 2 people. So you group best has at least 2 people. Makes training much easier.
-Gladiators (expect the eques) didn't use a tunic in the arena, so don't worry about the tunic color
(I don't expect you to do an eques impression, anyway). As for the subligaculum, just go for a color you like and which can be achieved using 'authentic' dyes. But most colors will do fine