I agree with the other comments: the fact that 2 different words happen to come from the same single root in a different language should not make them mutually taboo. "sign"/"signal" is not analogous to e.g. "book"/"bookstore" or "book"/"bookshelf", where the 2nd English word is directly built from the 1st English word.
If "car" was taboo, would you disallow "chariot" because they come from the same root?
If "host" was taboo, would you disallow "guest" because they (surprisingly!) come from the same root?
Well there was no latin grammar argument on the spot, we disagreed but continued playing of course!
To me they are clearly different forms of the same word, and went to dictionaries to research further, out of curiosity, and what i saw sort of matched what i thought. But i was interested in taboo rules interpretations, and i see i take them in a more constrained sense than other people. Oh well, for me the more challenge, the more fun!!