I have been playing 5 cloth classes (2mage 2lock 1 heal priest) last year and it is no problem till you get to the outlands. lockpets are simply not good enough at lvl 70 (let alone heroics). I dropped a lock and included a warrior for tanking. Today I whould go with a palading but back then I had a war available.
As for leading: warrior leads and is actively played, healer has his own set of keys (numpad of a nostromo) and dps is done by spamming one or two buttons.
Worked okay for me.