They do not in stock/BB lite/ RA 3/4/2.
The AM subs carry two also - but you only get two of these, can the be resized on a CS (like Chitose) , probably, and then you could put them on other subs.
Since you are playing RA 4.2 and I dont have that loaded in, I can't check in witp tracker - but you should be able to confirm if only those 5 units arrive and can convert by selecting aircraft / Model :TB / Seiran
Special : upgrade to or upgrade to path
none of the Glen units were listed
or you can select a Glen unit and check out its upgrade path in game or in tracker, Seiran is not listed
Size-wise I think the Seiran could fit sideways folded in a Glen sub hangar, but it was almost 3 meters longer.
Ps. And yes using the Mizuho to upsize FP units is considered cheating by some, you have to agree on it ahead - usually offer the allies a chance to upgrade their withdrawal units with Buffaloes and Peashooters and take the Lightnings/P-40s/P-49s out of them.
I usually like offering no withdrawals anyway. If allowed, by the way keep the Mizuho as CS. The Chitose class become useful 29 knot CVLs, the Mizuho as a CVL is too slow