Been so busy testing this I forgot to check back on this thread.
Yeah, I don't see where it would make a difference with the missions, unless you're driving a Gato in 1942 you're not gonna get the UserPlayerUnitType 7.Objective 1 mission, you'll get the mission for whatever type you're driving.
This is not actually working - started in an S boat, made three short patrols and was offered a new sub after the third patrol in January 42, as expected I got a Porpoise. Made three patrols in the Porpoise, offered new sub in April 42 - got a Gato.
[Flotilla 2.UserPlayerUnitType 7]
ID= F2Gato
NameDisplayable= Gato class
AvailabilityInterval= 1944-12-01, NULL
IDLinkUserPlayerUnitType= Gato
UnitTypeCommonality= 10
IDFlotillaLinkTransferTo= NULL
According to that a Gato is not supposed to be available for an Asiatic career until 1944, so I missed a file hack someplace.