I've never played with Primal Fear, but the wiki suggests they're still obtainable through eggs. They could be like Wyverns though, who have the "Can be tamed" boolean set to true to allow babies to be tamed through claiming, but have vanilla narco and passive taming disabled. Due to having "Can be tamed" set to true, it allows them to be tamed using the mod.

If the Indominus Rex is the same, they can just be excluded from being able to be immersive tamed by editing the settings via the journal.