"Where are you?"
Can I say "Gdzie ty?" (interpolating the fact that similar is valid and preferrable in Russian and Ukrainian?), or such phrase is invalid in Polish?
Well, AFIK in Rusiian and Ukrainian "jest/are" are often optional. That is not in Polish. gdzie ty? sound as correct as "where you?", or "where did you?"
Technically yes, but that's a pretty unusual construction, kinda poetic. You can remove the -eś/-aś ending (or other endings) from the verb and glue it somewhere else, I like doing it myself, but that's definitely a hiiiiigher level of mastering this language.
As another example, if the sentence is just constructed as a form of "to be" and an adjective, you can arrive at "Głupiś?!" = "Jesteś głupi?!" = "Are you stupid?!" ;)
But I'd rather not add such answers because they are simply outside of the scope of this course and if someone accidentally got such a suggestion, they would probably be very confused. I think you may be the first person that asked something like that.