Well I see where you're coming from, too - but hey, people mod what they want to mod. And in this case, it's a quick-and-easy mod, so why not? Certainly, one wouldn't expect a Marat to show up in the Pac, let alone play into an American sub campaign in any way. On the other hand, from what I recall someone did import a Russian cruiser class that was 'kitbashed' in SHIII that would be somewhat relevant. But again, people mod what they want. Some just want "their" ships in
(and I sympathise with the Marat-modder there, being Russian myself)
Otherwise, hey, it's an open field! Sure there's a ton of ship that are begging to be put in the Pac! For allies, the Dutch, Aussies and British are sorely lacking. More importantly, there is a TON of Japanese vessels that need to be made.
I suggest you check in the SHIII modding forum, specifically the Ship Modding sticky [thread]. There is a lot of good info and people would happily point you to the right tools. So far as we're aware, there's not really any more tricks to SHIV modelling than SHIII!