Which one do you want? image - Homeworld 2 The Dark Wars mod for Homeworld 2

To keep to the reality, the Beast actually doesn't have any destroyers, we know that Taiidan Battlecruisers could be infested and therefor Taiidan destroyers too, because Kushan ships are pretty much the same only with mostly different looks, I woudl say it's logical if the Kushan destroyer would be the one, playing Cataclysm you noticed that their capitalships couldn't be infested because by the time they got destroyers they already made sure to wash the infested decks with plasma from the engines.

I would like to see a Infested Somtaw destroyer but as you can see it's pretty unlikely but if the Beast got their hands on one then it should be much more powerful, you see there's a lot of space for the Beast to evolve in that ship unlike the Kushan version which is only logical that a Infested Somtaw destroyer will be much better than a Kushan destroyer even if their uninfested version would be pretty much equal in power and armor which they probably shouldn't be, you can clearly see that the Kushan destroyer looks much more fragile and have many more movable parts which should make it weaker automatically anyway making the Infested changes greater.
I think you should have both having the Kushan as a light Destroyer and the Somtaw as a Heavy, maybe even as a Cruiser class ship, don't forget that you may want to include the Missile Destroyer too

You should use both. The Beast needs all the ships it can get, and it would make sense that it's fleet is a motley collection of all existing ships from the HW1 universe. Even with the precautions the Somtaaw took, it's unrealistic to think that they managed to repel all infections that their ships suffered. They probably lost a few here and there. You should use both ships, but as the Beast would have a harder time getting at the Somtaaw destroyer, they should be exponentially more powerful and more expensive than the Kushan destroyer.

Really?? Then you've never read the OFFICIAL (As in the GAMES MANUAL) Lore, That by the time that an infection managed to get that far the crew had already overloaded the fusion reactors!! I mean come on, fighter pilots were adding self destruct switches and protocols to they're ships, I doubt a Cap ship would let an infection progress that far without blowing themselves up!!!

Also the Lore for this unit explains the Naar Protocol, The plasma flooding one.

I'll end my rant with this quote from the manual "From that day on, the Naar Directive, as it became known, became the standard response from Allied heavy capital ships to Beast infections. And, while the price in Hiigarian life is always high, instant death in a blast of high-energy plasma is preferable to the twisted immortality of becoming part of the Beast."

(Poor HW:Cataclysm manual, the only thing holding it together is the duct-tape I put on it...)

you could make the Somtaw destroyer to use it's own supply though that might be bad if you would like to spawn many Kushan Destroyers.