You are only allowed to have 1 pet at a time, so what happens if you use Helwen and target another pet. Does one of them just die, if so how is it decided which one, or do you get to keep them both since you didn't actually "summon" them both.

Also, how would that work for Kaal's effect of bringing everything into play from the graveyard? Would you only be allowed to choose 1 specific pet or would they all return since you are not actually summoning them.

What happens with Halwen is the following:
- You steal a pet
- You are now in controll of 2 card swith Pet(1) tag (regardless of hunter or warlock pet)
- You need to decide which one stays in play. The other one is destroyed.
- If you choose to Destroy Halwen, you will loose the ability that allows you to controll the other pet:

[exhaust symbol] -> While Helwen remains exhausted, you control target ally.

And that pet will revert to its previous owner.

Basicaly you can use Helwen as "pet removal" if you so choose

PS: regarding Kaal - yes they all come into play, but then PPP(pre-priority Proccessing) checks for uniqness rule, and forces you to choose to destroy any card that violates the rule. If more then 2 pets were brought into play, the rule keeps checking and forcing you to keep destroying violating cards - this all happens as part of PPP, no one is able to respond to this effect untill fully resolved by the controlling player. If both players are in violation of the rule the player who's turn it is resolves his effect first.

Hope this helps.

so according to the rules the player chooses one, and desotroys it. The way the rules read, it sounds like it would still come into play, so any effect of coming into play would still trigger, also any destroy effects would trigger. Is this correct?

If you are refering to come into play abilities like Zygore or Taz'Dingo, or Parvink - then NO - if you take controll of those allies with Halwen you will not triger those abilities, the allies are already in play. The rules state that uniqness is checked during PPP, not just when a creature comes into play.

407.5 Any time a player would get priority, pre-priority processing takes place first. This first involves a series of pre-priority checks (407.6). Next, any waiting triggered effects (708.1) are added to the chain. Then, that player gets priority.

No im referring to resurrection or Kaal's effect of bringing allies to play from graveyard.

For Kral those effects should trigger for the cards you mentioned as he is re-putting them into play.

However, Manowar was covering what would happen should a pet or unique with such a trigger effect be res'd by Kral.