In general, Spinoza and classical theists do not think God's omnipotence means he can create inconsistent worlds (e.g. he cannot create square circles); omnipotence (for them) means that God can do anything that is logically possible. For Spinoza, he thinks this means that only our current world is logically possible, because if there were other worlds that God didn't actualize, Spinoza thinks this would imply God's lack of omnipotence. For Aquinas, a classical theist, see Article 5 in this section of the Summa [0]. For Leibniz (not a classical theist, but having heavy disagreements with Spinoza), God could create other worlds like Aquinas, but unlike Aquinas, thinks this means that this world is the best of all possible worlds, which is why God actualizes this world as opposed to other possible worlds (Aquinas thinks otherwise - see article 6 of [0]).
For your edit, it's not clear exactly what "nothing is part of both" means in the context of God and his creation. I know it seems like I'm being needlessly difficult and pedantic, but the issue is that God is a being unlike any other. So much so that Aquinas would say that my latter statement is technically false - God is not a being, rather, he is the sheer act of "to be" itself. And so spelling out the relationship between God and beings is tricky---what exactly does it mean for a being to be part of/have being through/exist in/have being from something that isn't strictly speaking a being? Those prepositional phrases, when used in relation to God, clearly have a very different meaning than when we use them to characterize the relation of two beings, e.g. a leaf and a tree. That is all to say: classical theists may not disagree with "creation is a part of God" depending on how they interpret "part of"---if this means "creation has its being in God", then this is something perfectly agreeable to them.
[0]: https://www.newadvent.org/summa/1025.htm
For your edit, it's not clear exactly what "nothing is part of both" means in the context of God and his creation. I know it seems like I'm being needlessly difficult and pedantic, but the issue is that God is a being unlike any other. So much so that Aquinas would say that my latter statement is technically false - God is not a being, rather, he is the sheer act of "to be" itself. And so spelling out the relationship between God and beings is tricky---what exactly does it mean for a being to be part of/have being through/exist in/have being from something that isn't strictly speaking a being? Those prepositional phrases, when used in relation to God, clearly have a very different meaning than when we use them to characterize the relation of two beings, e.g. a leaf and a tree. That is all to say: classical theists may not disagree with "creation is a part of God" depending on how they interpret "part of"---if this means "creation has its being in God", then this is something perfectly agreeable to them.
[0]: https://www.newadvent.org/summa/1025.htm