Custom Pluging Entries FieldType I have 2 plugins I created that store similar data:

Plugin 1: id, name, panel_id, price

Plugin 2: id, name, test_id, price

I want to attach my plugin data to an entry, and I need to make sure they can only select one Panel or Test. I want to know if there is a way to add them as a source for Entries as a new field. Or can I create a similar custom fieldtype to be used for both plugins and have that kind of validation?

I'm using Craft V2.6
You can use a Relation Field for each of your element types and include a custom js rule so if one of them has a value, the other one can't be selected. 

The more difficult way would be to extend the field and the ElementSelectModal and the ElementIndex to select multiple element types but this is going to be a little bit more tricky since you'll need a custom controller to fetch all elements and can't rely on the ElementsController