Don indicated that complete information on plasma torpedoes will be in the Starships book later in the year, but I just knew there were Romulan fans out there, so I reverse-engineered the warbird stats from the NG to estimate what the offense values and space costs for the "Type V" plasma torpedo might be, and then made some guesses on Types I-IV.
I estimated space costs based on what it would cost to build up a similar offense value with a "conventional" missile weapon, so they are high; this is consistent with the BoP in "Balance of Terror," which if it did have space left for disruptors never actually used them against the Enterprise. The warbird in the NG seems to be working from the same principle; either that or it has quite a bit of unspent leftover space.
When calculating penetration for plasma torpedoes, use the offense values to determine what the point-blank penetration is as per Table 9.12 in the NG; then reduce penetration by 1 for each lower range category (yes, this can leave ships with single Type I and II plasma torpedo systems with no offensive power at longer ranges...but that's the price of ridiculous penetration at close range compared to ships of similar era with conventional missile weapons!)
I have no availability dates in mind for these, except that the Type III (the model I have in mind for the TOS bird of prey) must be available in or before 2266, and the Type V perhaps not until as late as 2360.
Size minimums are all my fault; change them at will.
Type I: Cost 5; Offense Value 7; Minimum Size 2
Type II: Cost 8; Offense Value 12; Minimum Size 3
Type III: Cost 14; Offense Value 20; Minimum Size 4
Type IV: Cost 21; Offense Value 30; Minimum Size 5
Type V: Cost 28; Offense Value 40; Minimum Size 6
This may, at least, keep somebody happy until the official story is told. Or not.