Picking up a Sig556 SWAT tomorrow. Is the SWAT rail interchangeable with the classic Swiss style handguard? I want to replace the rail and the folding/collapsing stock with a triangular style handguard and side folding only stock.

Yes the swat hand guard is interchangeable with a standard handguard, but it sticks in my mind that you also need the front handguard collar which means pulling the gas block that sits in front of it … it’s been a long time since I switched the one on my 556 pistol but I thing I found both the handguard and collar on GunBroker from seller Greenwood, he sells a lot of SIG & HK parts.

The short "patrol" length handguard is the one that requires one of two handguard caps, to be able to fit either a cut-down 550 or 551 handguard. If the gun is the longer gas system, you can drop in a US plastic handguard, or use a Swiss 551 handguard with a shim in the US block (if you don't want to convert to Swiss block). If your gun is equipped with the folding/collapsing triangle stock, a Swiss one will fit. The seller kimberkid is thinking of is likely gatewood supply.

i ordered from sig sauer USA, years ago a "classic" plastic handguard for the sig 556 pistol, near the end of that line. there is a handguard that sig made that doesnt require the handguard cap. finding them nowadays...is gonna be hard. it was a us made version. the slightly more available one is the handguard cap and cut/fitted handguard. the cap requires removing the gasblock to install behind it though... just FYI.
if its a one piece top rail. you will need a top rail replacement too.

I found a 551 handguard. Can anyone clarify what kind of shim I will need? It’s a US gas block.

Myself and others have successfully used a segment of zip-tie material, wrapped around the inner backside circumference of the gas block. I suppose high heat would compromise it; may want to check on it every so often.