Granting that God is the creator of all that is not-God, any other gods would just be creatures of God (like us). In short, polytheism is not really consistent with the existence of God as conceived by Catholics, since any other gods are necessarily 100% ontologically dependent on God. I don’t see why such entities should be called “gods” if they are fully dependent on another being–God–for their existence and causal efficacy.
Do such creatures exist? Well, maybe it’s possible that the Greek pantheon is sitting on Olympus, as long as it was created by God. Do unicorns exist? Perhaps we just haven’t seen them. But without evidence or argument, there seems to be little need to suppose that they do.
This response seems Thomistic in spirit, but I don’t feel like it’s cheating since the notion of a simple God on which all else is dependent just is the Catholic conception of God.
If you’re willing to indulge in a little Aquinas, I think you could formulate this argument:
1). Definition: An entity is divine if and only if the entity is a divine substance.
2). Premise: God exists and is divine.
3). Premise: The divine substance is purely actual.
4). Premise: If an entity is not divine, then it is neither a god nor God.
5). Premise: A purely actual substance is unique.
6). Excluded middle: Any entity is either divine or not divine.
7). If an entity is divine, then it is unique. (hypothetical syllogism, 1, 3, 5)
8). Therefore, any entity is either God or neither God nor a god. (constructive dilemma, 4, 6, 7)
Skimmed over a few logical steps, but the basic structure is there. 1). seems plausible to me. 2). is conceded for the sake of argument. 3). is a plausible Thomistic premise (it seems like someone would have to grant it if they were going to bother making an argument that Catholicism does not rule out polytheism). 4). is analytic.
5). is the important premise, but can be argued for reasonably in my opinion. It is analytic that a purely actual substance has no potentialities. Suppose there are two distinct purely actual substances. To be distinct one must have some potentiality relative to the other. But this is impossible. So any purely actual substance must be unique.