My thoughts on this....
If clan's are wanting to battle each other, they can have it hosted on a passworded server, where teams can be setup accordingly. As for parties in-game, it's not a great idea. Depending on the setting of the server, the teams are mixed in a few different ways. Complete random mixing or purposeful "team balancing" done by the system is the best way to have the game played. If there's 4 people stacking, they are most likely going to be highly coordinated and will have a huge advantage over the other team. I don't think stacking in itself is fair or good for gameplay. However, there are other ways of getting on the other team, such as requesting team change.
I don't think 'squads' or parties should be added to the game.