I understand that once you have a stand alone FAA certificate you can no longer apply for a 61.75 certificate.
§61.75 (b)
(3) Does not hold a U.S. pilot certificate other than a U.S. student pilot certificate;
Conversely if you have a 1.75 certificate and subsequently obtain an FAA certificate you do not have to surrender the 61.75.
It would be nice if this was not the case as you could hold both, then exercise the type ratings on your foreign licence in private ops without having to pass the FAA checkride for them.
As far as adding SEL to your FAA certificate it is probably easier to do a Comm rather than a Pvt checkride. There are less task areas to cover as you will get credit for having completed many of them as part of your ATP ride, but apparently this does not apply to a private ride.
I stand to be corrected if I have this wrong.