Boarding pass with partner airline

[Wingnaprayer]

Long time lurker, and first post:
I am flying PER SIN FRA ZAG. All on one ticket, as one booking, on SQ paper. PER-SIN-FRA with SQ and FRA-ZAG with OU. A redemption booking in J.
I am assuming bags will be tagged all the way through to ZAG.
However will I be given BP's for all three flights ex PER, or will it be necessary for me to approach transfer desk in Frankfurt for BP FRA-ZAG?
If so is it a OU transfer desk? And will this be airside?
Will I clear customs and immigration in Frankfurt as it is the first port of call in the EU, given that Croatia is part of the EU. Or will it be in ZAG?
Thank you for your help

 

[Lynda2475]

Croatia is part of the EU but not part of the Scheggen zone. When flying from BCN to SPU last year i had to clear immigration again even though flying from one EU country to another becuase of this.