You're not going to "mess up" the gun as long you shoot 12ga 2 3/4 "birdshot" or even "buckshot".
Doesn't matter if it's light target loads, #4 turkey loads, anything in between, or even not "in between".
2 3/4 12ga, that's all that matters.
Even slugs...but a slug bbl would be better.
For a manual on your 1100 go to www.stevespages.com
For cleaning.... check out www.Shotgunworld.com-how
to clean a Remington 1100
For serial number search..... www.Remington.com....or
call Remington, they will be glad to help.
Setting in the safe that long,a thorough cleaning wouldn't be a bad idea. Pay particular attention to the two gas ports located on the barrel under the barrel ring are clean.