The Standard Gauge Module Association specifies a certain carpet for its modules - you might want to check that out, it's something they have found works well.
If you are going to have 2 or 3 loops, they don't all have to be built with the same maximum diameter curves. My 2 outside loops are 84 and 72 curves, and the 3rd inside loop has some 42 curves - there's a lot of smaller Standard Gauge equipment that works fine on the smaller curves, and it allows more meandering - the big stuff uses the outside loops.
Also, bridges and other accessories that limit equipment size, should not be used on the outer loops where big locos and cars may be used.
Mine is a tinplate layout, but I didn't use carpet and I didn't paint gloss either. Flat latex painted Homasote - same simple technique used on the Carrail layout.
