Can I use Fusion gate to special summon more than one fusion monster as soon as I activate it? Also, is it still a good idea to keep polymerization in a deck for 2 fusion monsters?

You can continue to use Fusion Gate's effect so repeatedly fusion summon in one turn. But yeah, I think Fusion Gate is good in a fusion summoned based deck. Remember though that King of the Swamp can search out Polymerization, along with the soon to come out Fusion Sage. (Searches for Poly from deck and adds it to hand, coming out in DB2.)

Yes you CAN..................... try out a Metamorphosis over a Polimerization............ try to post this types of questions on the Rulings forum, you'll get quicker responses.

That was more of a gameplay question than a ruling question. Fusion Gate. being a field card, can keep popping in Fusion Monsters as long as you have the proper material monsters in play (hand and/or field).

Polymerization for 2 monsters... that's really something that you should test out yourself and see how it works. (The same goes for 99% of all questions like that) The things that work for some may not work for others, let alone all others. Give it a try against different decks and see what happens. Metamorphosis doesn't have much of a point, especially if you want to be able to get your big fusion monster back from the grave.

Give it a try, see for yourself. Have fun.