I flew PG from LPQ to BKK in January. Although, I proceeded thru immigration, I remember that there was an air-side bypass lane for transfers/connection to other airlines. TG planes were parked next door.

Interline agreement? I don't know. If you can't interline, baggage claim is beyond immigration check. PG is a very nice airline. Full meal service with wine from LPQ to BKK. Flight attendendent saw me staring at the bottle of wine in her cart, and she thoughtfully picked it up, spun it around and presented it to me so I could read the label. (I alreadly had a full glass, so I obviously did not need more.)

Coming the other way thru BKK to Chiang Rai on TG, I was able to do the airside transfer in under 10 minutes without clearing customs in BKK. (Admittedly, everything went perfectly and a more typical time may be 30 minutes if you know where you're going.) Cleared customs in Chiang Rai. I "could" have bypassed customs in Chiang Rai to enter Thailand, but if I did, I would have had some explaining to do (and a fine to pay) trying to exit Thailand without an entrance stamp.