I know your priority lies on pvp but I still recommend having one or two warlocks as tanks for pve instances. Maybe put them on separate accounts so you can have 8 mages 2 priests and 1 or 2 locks to add. That way the mages can take turns when it comes to instances.

Or, and this is what I do although it is going to take me a couple of months before I can complete that, is have a warrior on a separate account for pve tanking. I assume that will be pretty good when trying heroic instances. For pure farming of non elite mobs he is left out of course. You can also try 10 mans with a warrior. The setup I am going for at the moment is

Group 1: priest, 2 mage, 2 locks (currently lvl 63)
Group 2: war, healadin, mage, 2 shams (currently lvl 23)

When they are lvl 70 they will be transfered to 10 accounts so I can put together my groups depending on what I want to do. Even a decent pvp group (e.g. healadin, 3 mages, 1 lock) is possible.

Have fun, Bunny