Two things:

1. Rules (REQUIRED, etc) must be defined on a field; you cannot define it on a form control. Therefore you cannot make tab required - but you can definitely make required a field on that tab.

2. We do not support rules on links or link counts, therefore what you want cannot be implemented directly. As a workaround I can suggest adding two fields to your task type for storing id and type of the linked item; make them required so noone will be able to save a task without setting these fields. It won't be a link, of course, and you won't be able to validate that data, but at least you'll have some validation.

Thanks,

Alex