The reason that I really dont think that are more Sivars is that the Emperor did not like blowing planets, his stategie was to take planets one by one until they reach Earth, the emperor son made the sivar to make a full attack on Earth, since he fail and the Emperor never have any faith in the Sivar dreadnought (getting the prototype blow up is not a good first start), I belive that was droped, the PTC was a modified version of the sivar weapon, its does not have the power that the sivar but it can kill a ship in one blast, but it does have those problems that lead to all PTC being removed from the Confederation Class.
Even Trak did make a mistake, the cloaked fighers were developed on that rebel Kilrathi planet, forcing him to destroy the bases were they were developed, it have taken 10 years for the Kilrathi to get more cloaked fighers.