Are the two above interchangeable? Any issues with the gas piston assembly if it were done.

I don't own a SAN SIG 553 and so I can't say this with %100 certainty, BUT, the SAN SIG 552 uses an AK style gas system with the action spring mounted aft of the BCG and also has the piston fixed (pinned) to the BCG. The SAN SIG 553 gas system uses a forward mounted removable piston and action spring. With these differences I don't see how the two different Bolt Carrier Groups could possibly be interchangeable, at least not without major modfications.

However, The SIG P556 bolt, which is the same configuration as the SAN SIG 552 bolt and is interchangeable. Again, I don't own a 553, but I think the bolts of the 556 and 553 are the same and should be interchangeable as a stand alone component, and therefore the 552 and 553 bolts should be interchangeable.

Although you didn't pick a wrong place to post your question, you might consider asking the mods to move this to the rifle forum where it might get more views and, perhaps, a more definitive answer than mine.

Welcome to the forum!

Thank you, Sir.