Hi,

there is no chance that a Felguard or voidy can tank a boss in heroics. Even with a paladin you need a own healer for them, because he has no time to head itself (and will be oom within 1 minute)

I would suggest to have a Prot Pala or Warrier (or Feral Druide, which keeps much better Agro and gets much better suite in normal/heroic instances than a pala/warrior), than one Priest for healing (group healing), two mages and one warlock, because of some abilities of a warlock. Than you have two cc with mages and one cc from warlock if you have demons in the groups