I would make Mongul & Mxy CGI.
Do not like the Rock casting for Lobo! Why--because he can raise one eyebrow very high?! Forget it. He is not a good actor.
For Bizarro--I would go with Henry Cavill...why not? he is Superman but in Bizarro world or a Bizarro version of himself so use the actor that plays Superman. let's see his acting range when he does that.