Scully,
The choke is a constriction in the bore at the muzzle of the gun.
Replacable screw-in chokes are a relatively new developement.
In the old days, the choke was machined into the bore when it was made.
Many shotguns are still done this way, with a single fixed choke.
The barrel should be marked near the breech end with the choke and gauge for the bore.
Look for "Full", "Mod", 'Imp", or "Skeet" stamped on the barrel.
It is possible, but expensive to have removeable chokes fitted to the barrel.
Probably better to buy a new one ready to go.
Yes, you can shoot slugs out of an unchoked barrel with no problem.
A Skeet or Imp(roved) choke shoud be OK too.
No to Full or extra full chokes and I'm really not sure about Mod(ified).
The best thing is to find out what the choke is on your barrel and check with the barrel maker for guidance.
You did good, the 1100 is a great shotgun and will give you many years of reliable service.
Jack