Mass Polymorph -> Kederekt Leviathan

[vanguardian]

I just saw a forum where somebody posted the following question:
If I Mass Polymorph into a Kederekt Leviathan do all my creatures get returned to my hand, save the Leviathan?

The question had two responses. One guy quoted some rules which really didn't help and concluded that yes, your creatures you just Polymorphed into will be returned. The other guy just said something about Stifle.

So this is a good question. I know that whenever Mass Polymorph finishes resolving all the creatures' go onto the stack at the same time. Its the "Scapeshift/Valakut" thing. But, the creatures are indeed in play regardless so whenever all the other creatures' "enters the battlefield abilities" finish resolving they will all be returned to your hand. Unless you choose to return them to your hand first?

Is this clear? and is this correct?

[vanguardian]

Oh, and do we need a section for Rules Questions?

[natedog08]

All your other creatures would be returned to your hand, but they're ETB effects would still trigger. And, a Rules Question section might come in handy.