Slighlty different rules, i think they just a have no more that 3 Non EU players rule (If i remember right).
Vincinis was registerd to the B Team, and still eligible for the senior side, therefore not a taking spot last season.
The same CL rules apply though.
And most non-eu players take up eu country passports as their second citizenship. So doesnt really slimit anything much.
Valid point on the B teams, big advantage and that is how Barcelona can boast such a big Cantera kid quota (one for the old times).