Is this comment that I made here grammatically correct?
In Latin, when a group of males and females is combined, the neutral plural form is not used, but rather the masculine is.
'but rather' is perfectly fine and a good alternative to:
'Rather' is like 'instead'; using 'but' allows it in one conjoined sentence rather than a separate one.
Assuming you are referring to the clause "when a group of males and females is combined" then the answer is "Yes, it is correct." Because you are talking about a group (singular), the verb should be is. Were you to restate it: "when males and females is combined" it would, of course be wrong, and should be written: "when males and females are combined" because you are using a plural subject.