I think if you're going to have the station in Barrie itself be Allandale, then the "Barrie South" should renamed to something else. For there to be a "Barrie South" there's gotta be a "Barrie" or a "Barrie North". "Barrie South" always struck me as a temporary name. Several GO stations have had such temporary names (e.g. Oakville West I believe is one).
As for needing one of the stations to have the name "Barrie" in it, I don't think that's necessary. The name of the LINE is Barrie. If anything "Barrie" anything on the "Barrie Line" is redundant. Mississauga doesn't have a single station named after it, instead they're named after neighbourhoods (Erindale, Streestville, Cooksville, Clarkson, Port Credit, Malton, Meadowvale) or streets (Dixie and Lisgar).