MicrostockGroup

1: If you choose to be exclusive (with any agency) you agree not to sell via any other outlet. So I would strongly recommend not to try to break that agreement on the hope that they will not find out - because sooner or later they will.

Whether you choose exclusivity or prefer to sell via multiple agencies, that's a personal choice that is debated regularly at the forums with supporters for both camps.
It depends on a lot of things, but for myself I can only say I am not considering exclusivity. I rather not put all my eggs into one basket.

2: Right, you get (as a non-exclusive beginner) 30% of each sale at Fotolia. If a file sells for 1 credit (smallest size) you get 30 cents.

As Royalty Free.  You can sell images - other images - as Rights Managed.