IMO the more the merrier. My only concern is that there aren't too many classes of large tankers (between, say, 9000-14000 grt) since that would skew randomly-generated encounters and inflate the average skipper's tonnage. Like the Scandinavians the Japanese tended to produce beautifully streamlined tankers, especially during the 30's and 40's - it would be a real treat to see them in SHIV.
Regarding older subsims, I have always liked the AOD merchant models and tonnage system, which were innovative for their time. SH2 was hit or miss; the T-2 was excellent, and the generic tanker OK. I couldn't stand the fast cargo-why on earth would you put the bridge behind the funnel? The slow cargo was OK but not a good stand-in for medium freighters of the era because of the big superstructure (like passenger ships).
The vessel feature randomization idea is excellent - it would add tremendously to replay value. With a little work there could be rules inserted so that all the features would look appropriate - for instance, tall, thin funnels and sparse bridgework on older ships, shorter smokestacks on modern motor vessels, and so forth.
Unfortunately based on the screenshots SHIV seems to focus for the most part on "exciting" vessels like battleships and heavy cruisers. This isn't appropriate b/c even in the Pacific the main goal was to starve the home islands into submission, not to hunt down individual fleet units. In fact, by the end of the war many of Japan's largest warships were essentially worthless b/c of poor crew training, accumulated battle damage, and (especially in the case of carriers) inadequate aircraft. OTOH, the larger merchant vessels from Japan's major shipping lines (Daido, NYK Line, Yama****a Line) were worth their weight in gold THROUGHOUT the war.
IIRC there are to be 70 combat units in all for the game (air and sea). Considering all the planes and warships that need to be included, I fear we're going to end up with a small, repetitive sampling of merchant ships.
AOD's over a decade old and we're still stuck with "Big Merchant, Average Merchant, Little Merchant". In fact SH3 was a step backward from AOD in that there was no tonnage randomization for generic ships.
Anyway, enough whining from me.
Hopefully it will be easy for SHIV modders to add new classes.