They can "reject" you insofar as your spiritual constitution does not resonate with your choice of symbolism/archetype images. This usually manifests as a feeling of "I'm not fully connected/can't fully get into it", prayers or invocations feeling really awkward or as if they stick in your throat, deities appearing sporadically when invoked instead of the specific Godform you are calling to (usually a sign that whatever archetype you're most prone to invoking via resonance may not be so explicitly found in the pantheon you're pursuing, but it can be other things too), and so on.
Divine rejection is not like human rejection. Isis, for example, is a channel for an objective archetype whose name is unspeakable, and she is found throughout cultures in different forms... for a reason. Certain channels resonate with certain constitutions more than others. So, divine rejection is literally just "hey man, these archetypes-as-channels you're interacting with ain't the best for ya so things just aren't clicking quite right".
So, yes, there is an objectively "most harmonious" pantheon/set of archetypes and symbols an individual resonates with. Usually you get hints from symbols you get throughout life. I lean very Egyptian, though my bloodline is far removed from ancient Egypt (I have like 0.3% Coptic Egyptian in me though, seemingly that's enough lol), and the way I knew this was from visions I got early on in childhood or teenage years, where the Godforms came to me under Egyptian guise before I even knew what or who they really were. But I understand most people don't have such explicit signs. Still, even small signs like "I lean more solar" or "my birth chart has this particular influence" can help you triangulate something.
In my opinion, as long as you approach with honesty and a real desire to know and love the world, you'll find your way eventually. Ego and going by its whims is what makes you lose your way most in this regard.
P.S: Pick and mix as an approach can cause more confusion, especially if you're not quite sure how harmonious the geometries of the two or more pantheons you're trying to mix is (in the way that you're trying to mix them, anyways). Syncretism is a rather delicate operation, much more so than most people believe it to be.
