You can have up to three species in one lagoon depending on the combination. I've had Ichthyosaurus, Attenborosaurus, and Mosasaurus together (eight, four, and one respectively) in a 12-section lagoon with six fish feeders and the mosa's two shark feeder requirement, and they never harmed each other. As long as there's plenty of space and enough food, compatible and neutral animals should coexist peacefully. I'll post the seven animals' stats and Likes/Dislikes to help.
Attenborosaurus: Medium size, likes Small animals (Ichthy and Plesio), dislikes other Medium animals (Elasmo, Lio), neutral towards Large animals (Mosa, Tylo)
Elasmosaurus: Medium size, likes Small animals (Ichthy and Plesio), dislikes other Medium animals (Attenboro Lio), neutral towards Large animals (Mosa, Tylo)
Ichthyosaurus: Small size, doesn't have any Likes, dislikes other Small animals (Plesio), neutral towards everything else
Liopleurodon: Medium size, likes Small animals (Ichthy and Plesio), dislikes other Medium animals (Attenboro, Elasmo) and Tylo, neutral towards Mosa
Mosasaurus: Large size, likes Small animals (Ichthy and Plesio), neutral towards Medium animals (Attenboro, Elasmo, Lio), dislikes Tylo
Plesiosaurus: Small size, doesn't have any Likes, dislikes other Small animals (Icthy), neutral towards everything else
Tylosaurus: Large size, likes Small animals (Ichthy and Plesio), neutral towards Medium animals (Attenboro, Elasmo, Lio), dislikes Mosa
So you can have one Small, one Medium, and one Large species in a lagoon, given enough space. 12-15 lagoon sections should work best, and make sure there are a lot of fish feeders, especially if you have the max of each clutch of eggs in the lagoon. Something else to point out is that Ichthy and Plesio will panic around Liopleurodon but be calm around Attenboro and Elasmo, since the latter two are incapable of eating them, so for best results give Lio its own personal lagoon.
Addendum: I finished determining the lagoon size of every species, so this might help you determine the size of a cohabitation lagoon. This was done with one of each species, if you have more of each add 1-3 extra sections (disregarding Ichthyosaurus, who even with max 8 individuals is still happy with 3 sections):
Attenborosaurus: 4 sections
Elasmosaurus: 4 sections
Ichthyosaurus: 3 sections
Liopleurodon: 3 sections
Mosasaurus: 5 sections
Plesiosaurus: 4 sections
Tylosaurus: 5 sections
This means that if you want, say, 8 Ichthys, 2 Attenboros, and a Mosasaurus together, you should make a lagoon with 12-13 sections. Have around 5-7 fish feeders scattered around the rim, and 2 shark feeders where the Mosasaurus claims its territory. This should ensure your aquatics will be happy and refrain from harming each other.