SignIn to Youtube (Android App) with Gmail but without adding Account
You tube is Google owned and therefore, is closely related to the purchases you make, your viewing habits which are monitored by Google (read ad and revenue generation). So, the tight coupling is inevitable as answered here
That leaves you with the option of signing out of Gmail  (of the youtube related account) or  use alternative approaches like FireFox addon or IMAP approach so that you have mail access as also youtube stand alone


It's a workaround