pet tanks for 5-boxing?

[Stealthy]

I don't think the Owl AoE debuff stacks, so just one will do ya. (I could be wrong.. never actually tested it. My original dwarf hunter had an owl for over a year... "Peckerhead" lol )

Bears have been stated to get swipe in WotLK, which will hit three targets. That will go a long way to hold aggro on additional mobs. But this won't help you now....
Gorillas have an AoE thunderstomp ability, which would help as well (lowers attack speed). They only eat fruit though (pain to feed).
Wolves have the AoE AP buff, which means more DPS for all pets, which means more aggro.
Scorpids get poison DoT debuff, which stacks. Again, goes a long way to holding aggro.

If you're BM spec'd, all pets do considerable damage, so I'd focus more on abilities than DPS output. Plus the serpent swiftness talent in the BM gives a nice overall damage increase to both hunter and pet.

Now with that said, I've had a lot of success with 5x cats. I just send them all in, and tab target + pet attack spam all mobs in the group, so when one dies, they automatically pick up the next instead of returning to the parent hunter. Drop a mend pet as they run in, so you don't get healing aggro.

Having a healer in the group would make things insanely easy... I do it the hard way without a healer (even though its not really hard).

So if I was motivated to build a PvE "pet team", I'd probably get:

Owl
Wolf
Gorilla
Then two DPS pets. (for 5 huntards)

FYI - Owls are losing the aoe debuff in WotLK. The ability is going to carrion birds. Owls are getting a disarm ability.

Screech doesn't stack - thats true. Unfortunately different ranks don't stack anymore either (i have tested this). However the screech debuff still generates threat against all mobs within melee range each time it's cast - and this is what makes it so valueable.

For WotLK - IMO bears will probably be the boxers choice of tank. Swipe only has a 10 sec cooldown and damages 3 enemies. Thunderstomp (Gorillas) has a 1 minute cooldown - good for an opener, and thats about it. Furious howl (Wolves) gets a range increase (to 20 yards, up from 15) - which is good, but not nearly enough IMO. Currently in practice your toons will only see the benefit from Furious Howl when you first send your pets in. After that, most likely your toons will be outside of the 15 or 20 yard range and won't get the buff (your pets however will).

However that being said, I think having a mix of pets (2 or 3 tank pets and 2 others) will be the best bet.

Cheers,
S.