Troll castes are a biproduct of them evolving past eusocial living, think like ant differences in anatomy between a worker and a soldier. This explains why they’re all compatible but so, so different. Mind powers like Vriska and Tavros’ are uighly evolved versions of how they used to communicate when they were eusocial, but phased out when they became independent as it caused infighting. Now it only exists as a rare mutation.