Wht do we say "Sum puella" ? Can't we say "Ego sum puella'?
It's important to understand that you don't need the additional pronoun (ego), since the verb sum expresses "1st person singular subject" ( = "I") in itself.
It's also important to understand, as the commenter CF PLUS puts it, that the word order of "sum puella" also doesn't matter. Puella sum also means, "I'm a girl."