Necro pet can absolutely tank, but there's a caveat to it. You need to be running the best Raid level EM earring you can get... in your case x 4.
My advice is to put 1 mage in the party, 3 necros and the druid can buff / heal / port everyone

Only reason being, then you only need 1 raid level EM earring for the mage and necros can just use highest group EM item for added DPS.
Edit... Then you also have baked in pet toys and COTH for what that's worth (huge time saver)