I don't know how successful that's going to be, RedFlag. Arena maps are designed with the purpose of small, localized combat in mind. I wouldn't even know where to begin in placing capture points. I think it just doesn't have enough room in it to do what you want.
CTF, however, seems kinda possible. You've got two bases and a central area. Yes, the gameplay would be fast and vicious, but It might be worth a shot.
As for removing arena mode, Rogue sniper said it best. Just find the entity either in the map or by using the find entities tool (and yes, I'm pretty sure it's info_gamerules.)