I think we can easily limit it to primary species only. We may even be able to create a policy choice which determines it, in fact.
Regardless, atm it picks the POP you recruit at random, which makes using it as 'special purge' is tricky anyway.
Policy would be good. In the base game you recruit armies based on which species you recruit from; shouldn't the pop used up reflect this?
Also (unrelated) would it be feasible to implement some kind of experience mechanism for actual units? (I'm thinking of battleships mainly; capital ships not escorts).