Slave countries in Federations

Hi everyone
Can somebody tell me if slave countries are allowed into federations?
I have included my slave countries into my federation but been told it might not be possible to add somebody else's slave countries into my federation. Is this the case? Is it against the game rules? Can't say I have seen anything relating to this matter. Thanks in advance

I do not remember any limitation on membership except for a max number.

I do not think we know the exact status at the time of joining the federation.

If you bump into a limitation, please let me know.

there is only one limitation of max countries to be in fed(25), no other limitations

Thanks for the clarification