I like the size of your layout! Lots of room for additional items. Not a big fan of cats - seems to me that there's a better place for them to be than on a train layout... especially with green felt!
I have a small O gauge tinplate layout, and just (York) acquired some Standard Gauge tinplate so I'm setting up an under-the-tree layout then will use some of the panels for a slightly bigger one in the basement.
I used thin grey carpet on my first layout and will do the same for the second... then grey is neutral, and brings out the colors of all tinplate stuff! Not that expansive at about 65 cents per square foot at HD, and in 12' x whatever size, there will be no seams. I highly recommend this stuff.