That seems like a reasonable assumption. There is not evidence (that I have seen in my reading as an amateur looking at the period), that any Greek city states used standardized badges or designs. Even the equipment was varied- archaeologists classified helmet types based on where specimens were initially found, but that doesn?t seem to correspond much with how prevalent they were in use. Later there do seem to be some types that were regional, like the Boeotian helmets you mentioned, or the conical pilos helmets the Spartans used. Even then, that characteristic equipment was probably more widespread.
For Plataea, Theban likely looked indistinguishable from the Greek hoplites fighting against them.