Yeah this is an interesting concept. You'd probably need to build a new provincial campaign map (or modify the Prologue / Sons Of Mars map), focusing on the Italian penninsular.

For individual gladiator units, you have numerous approaches. You could strip all vanilla units from existing faction rosters, and put individual gladiator units under each vanilla faction. Obviously you could then rename the vanilla factions to be a specific type of gladiator.

Alternatively, as snevets has suggested, you could use the vanilla factions as gladiator training schools who compete against each other. Each school could be restricted to specific gladiator types.

I'm not sure how you'd go about sorting out the actual combat environment. Perhaps siege battles (IE the settlement) needs to be replaced exclusively by a scaled-up colosseum venue.

You also need to think about the linkages between the campaign map and the battlefield, such how would specific types of gladiators be able to fight each other at an arranged location. Perhaps diplomats can be used as messengers to offer gladiatorial combat between rival schools? Unless of course as mentioned, you focus exclusively on custom battles, as this will allow you to skip the pre-amble on the campaign map.

Anyways, an interesting concept