I hate mixed torpedo armament for exactly that reason
In the beginning of the war I tend to rely on atmospheric torpedos and only carry a single electric one in tube 2 (and tube 6 in a type IX boat). No 4-torpedo salvos at high speed, but I like the ability to have a stealthy shot when I need it.
Shame we can't borrow some torpedos from our Japanese allies... they used pure oxygen instead of compressed air. Fast, long range, and since there is no left-over nitrogen after the combustion process no bubble trail either.