This is going to be handled exclusively through e-mail, right? I've been thinking a bit about how players would get matched up. Do your monster and girl form a team to take on the other moster and girl, or does your monster and girl have their own seperate matches? Requiring a player to field one of each makes sense in that if you didn't there might be an odd number of pairings (too many monsters, not enough girls).
You could have the central program make all of the pairings according to relative scores. Either basing them on overall value (logical), or on win record + flare modifier. I think a method that takes both into account would be best. Create leagues that are based on a certain minimum number of wins and have a reqiured overall value for your team or they get bumped down a league until they make the value requirement too.
What could drastically simplify this would be if the players could pick their own matches. Based on some kind of ranking system starting from the top player down each player can choose the next round's opponent. If you get chosen before you pick someone your locked into the battle so you don't get a choice. I'd say start 3 days before the actual match and unlock each player in order with a certain amount of time between each unlock. Like say 1 hour per person so a full day after the first unlock you've probably got your entire leauge unlocked. If for some reason you want leauges bigger than 48 people have them unlock faster, like 1 every 30 min or even 1 every 15 min.
I think that might take some kind of accessable website, or maybe it could be handled through e-mail still. If it had real time updating then your talking about a large volume of e-mail. I'd suggest at that point you require all players to make a dedicated g-mail account so they are consistant and have the client auto-save and auto-delete game related e-mail.
Also I'm not sure how much info your going to give players about the other players. Having no info posted on how the other girls and monsters are doing would leave way too much to chance. Giving all the info would basically allow people to predict with way too much accuracy the outcome of fights. I'd suggest 2 systems for gathering info. First an official bio that the league itself posts. Gives the player name, and a short bio on each gladiator owned by the player. Win record, match record, weapon used in the last fight, and maybe some kind of title based ranking system so you know generally how powerful each gladiator is considered.
Then have a secondary system where you have to actually do something. Either you can spend some of your time spying on a single other player to gather info on them, attend social events where you can gather info on every gladiator attending, bribe trainers and coaches for info, or hire actual spies to watch other players and their gladiators. Maybe a rumor system where you get info for hanging out in bars?
I'm not sure if players should be able to pick their teams before or after seeing who they are going to be matched up against. I think after might be more exciting, as it adds more risk in the actual pairings. Also an experienced player could throw a match by sending in their newly aquired gladiators instead of their top ranking team.
Some kind of handicap system needs to be in place. Like if the match is very unballanced the winner doesn't gain any experience from the fight and the loosing team gains double. Having the higher rank team get punished probably isn't desirable if the players can choose the actual teams after the pairings have been made since a high ranking team could send out low ranking gladiators to throw a match. Maybe instead of a combat match, it become a flair contest to see if the winning team can keep the crowd entertained despite this being a one sided match? If they succeed, the strong team still gains flair despite not getting any other kind of experience?
Anyways, looking forward to this.