If you was untagged, then grabbed by someone, then tagged - you must retal before tagjump or you can do it after you join clan? Or it does'nt matter?

Unless your current tag has a pact with the tag that hit you, you get to retal
Best check with your FA before

well, you can hit but you definitely can't accept FA or any sort of aid or assistance from your new alliance to make a retal on someone who hit you when you were untagged. imo that's the only thing you need to avoid.

and if it's laf that hit you, you are not allowed to retal :p

I understand that this is frowned upon in general, but it certainly isn't prohibited, unless pact terms dictate otherwise.