The reason some people keep complaining that the pirate SRA ships keep showing up so often is because, in an otherwise unmodded game, they are. The msp_potniaBis_FS and the ms_clade_P_FS have 10 weight each in the pirates.faction escortMedium role, and in a vanilla game there are NO ships in pirates.faction flagged to fill that role. So whenever the fleet generation code making a pirate fleet looks for a ship to fill the escortMedium role, in a vanilla game it falls back to a combatMedium ship. In a game with SRA only it'll always use a SRA ship. It doesn't matter how low you set the weights, those two ships will just keep showing up all the time unless they're removed from the escortMedium role.
Mods that add ships to the pirate escortMedium role will dilute the concentration of SRA ships in this role, so I think this is why different people have different perceptions of how common SRA pirate ships are; they actually are experiencing drastically different amounts of SRA pirate ships, depending on whether or not their mod setup includes mods which add ships to the pirates escortMedium role. For example, Underworld adds 8 Shark variants to the pirates.faction escortMedium role, with 1 weight each, and BRDY adds 2 silverfish and 2 zabrus variants, with 1 weight each. Dropping the two SRA ships in that role down to 1-3 weight each would bring the proportions in line with those two mods, but still won't help in the situation that someone is running a game with SRA as the only mod.