I'm also playing in a game that allows a myriad of classes from all sorts of D20 games. I'm going primarily with the Star Wars ruleset. I modified all the non-SW classes to get defense bonuses as they level, as well as changing their skill selections. There have been other modifications I've made to some spells and powers to make them fit in more with the kind of game that I would like to play. So far, it has ran pretty smoothly. I have a Cerean Samurai, a Dhampir (Half-vampire from Ravenloft Denizens of Darkness) Soldier/Bounty Hunter, a high elven jedi guardian/cleric of Elhonna, and a noghri Codan Warrior (a modified Diablo 2 Amazon). With the VP/WP system combat is pretty lethal, and many weapons deal large amounts of damage, this leads to an evening out of the playing field. We noticed that sorcerers and wizards are extremely hard to keep alive in a firefight at early levels, but as the party found out the other night, higher level magic-users with the right feats can be extremely tough.
Throwing it all together without modification, IMO, will lead to a great unbalancing effect. The various system (with the exception of D&D and OA) are not truly meant to be ran together although they do use the same core system. Therefore, if you want the classes to be somewhat close in power with each other you need to do a little work to bring them together under the banner of one system or another (like I did by making all the classes SW classes). Sure, it was a lot of work, but I'm having more fun now than I think I ever have in a RPG before!
Adios,
Kane