Shonisaurus seems like the Triassic Ichthyosaur they'd probably gravitate towards, though Cymbospondylus is a plausible companion, being smaller and potentially moving more snake/eel-like.
There's not really an issue with telling it apart from the Mosasaurus, the size difference is huge for one, though I could see Cymbospondylus being too similar to smaller Mosasaurs depending on what they want to do.
I don't see them adding Nothosaurus... possibly ever. Being amphibious, I'd assume they'd be considered land animals (the lagoon appears to be for strictly aquatic animals) and when it comes to amphibious options I can't see them picking Nothosaurus over something bigger... like Deinosuchus or maybe Sarcosuchus.
I don't know about being land animals, floating islands seems the more likely approach (
considering land animal swimming doesn't seem like a thing, and the majority of their time is spent in water), in any case it's almost definitely a post-release problem, should they choose to tackle it.