Frix’s mistake her is telling Sydney it’s probably okay to mix and match. But then, they’re not trying to fool people who know them, or even anyone who even knows who they are or are off-planet. This is all assuming Sydney doesn’t want to try being a guy for the next however long this trip is going to take. Granted it adds a layer of complexity to a situation she hasn’t exactly trained for in the first place. Again, they’re not exactly trying to infiltrate German occupied France, but best to keep odd behavior to a minimum.