Holy nova consumes a lot of mana, you wouldn't be able to kill heroic mobs fast enough before running OOM with it.
Spriests are easy to gear up, just level up tailoring on all of them and you have a fully decked out character at 70, that will only need some badge gear.
Heroics are going to be hard no matter what, imo. Mixing too many classes means you won't get peak efficiency (for example your resto druids would be mostly keybound to heals, and probably not be able to cyclone/root fast enough loose mobs). Spriests have very, very good DPS in heroics/T4, it only drops off relative to other classes in the high T5/T6 levels. With a pally tank plus 4 VE's from priests in non-shadow form + 4 quick heals you should be able to do most heroics. I think places like heroic MagT, shadow lab, shattered halls will give lots of headaches but thats true for any multiboxer really.
4 holy priests would be similar too, although imo the DPS gains from shadow outweight the heal gains from holy - much of the holy talents are about maximizing HP/s, and in heroics the trash is usually the hardest part, and none are fights longer than 5 minutes, so maximizing efficiency not that important. You will probably want a pally tank though, just being able to aveng shield, consecrate and getting back mana from heals (including VE) is massive synergy. It'll be a lot easier having the other 4 be the same class, rather than trying to mix it up.
Think more about all the times when stuff goes bad and how you'll react to that - DPSer pulling aggro, tank getting low on health, pulling adds, mobs running away, etc. Invariably its a ton easier with a pally tank (few keys to tank, can tank a ton of mobs, mini shield wall at 35% health) and 4 others who are all the same class and same key binds.
