From Dragon of Desire:

Scape and Run parasites has gotten a LARGE amount of updated mobs and work. You use 1.9.1, and the mod is now at 1.9.12. I tried adding in the new version by hand but it caused the Rupters and the maggots to spawn to the point my game lagged, so there's work that needs to be done there if you want to update to the new version. There's been a ton of changes, and I recommend it, including a corrupted ender dragon.

SRParasites mod completely redid the config file, so that's probably why simply updating the mod without updating configs has incorrect spawn rates.

Also, you really should allow the parasites to infest zombies OR strays (not both) so that the progression is more natural.