I haven't found any info about Praziquantel being harmful to corys, I found this quote on one site
What it's good at: Because it's gentle, Praziquantel is, and
always has been, the "treatment of choice" for Flukes
(trematodes) in fish. Praziquantel does not require water
changes, praziquantel is harmless to fish of *all* species
including Discus, Koi and goldfish fry, and is non toxic to
plants, and; (unlike Formalin), praziquantel has no negative
filter impact.
Also some info in this link
http://thegab.org/forum/viewtopic.php?t=6445&postdays=0&postorder=asc&start=0
Cheers
Gemma