As I see it, there really are only two or three aspects of this guild idea that I see as being a potential problem... Only thing is, those ideas very easily can be ones that polarize the PS community and (rightfully so or not) overshadow any of the other arguably good ideas.
So, since I've already pointed out what I personally see as problematic... I guess the least I can do is offer a few thoughts that I think could potentially remedy some of the more controversial aspects.
1. Take "donations" instead of charging fees. As it stands right now, the fees are so minimal anyway, that I'd guess that you'd probably make more money from kind benefactors that like what you're doing, enjoy the tournaments, like having "real" gladiator training partners, than you ever would from charging minuscule fees. For example, one person donating a small gift of 4 circles would be the equivalent of 50 people that had paid entrance fees.
This would also eliminate any possibility that a person could feel "railroaded" by being asked for an entrance fee, and removes one of the controversial RP issues.
2. Reconsider the "we are official" vibe... I have no problem with an organization that works for the improvement of the arena and the promotion of it's facilities but, when you try to tie in having Gregori actually running a player based guild... it seems to at the very least push the boundaries of what is acceptable RP. If a group was operating in the arena, obviously they'd be doing so with Gregori's permission (from what I've read in his book, he'd probably allow it as long as he got a "cut"

). However, why is it necessary to be -The- Arena staff? If this guild was an entity seperate from the actual NPCs that run the Arena, it seems to me another possibly controversial problem would be resolved.
3. Don't worry about assigning NPCs... It would be more of a headache than it's worth for reasons too numerous for me to even want to attempt listing. (Keep the assignment idea though with the player gladiators, I think it's a good idea and could work well.)
And with that... I think I've probably said all I've wanted to say. Hope some of this is actually helpful, would hate to see a handful of questionable guild aspects overshadow and potentially doom the many other awesome ideas you came up with for this guild.