You can if you want to. If you're having problems with bullying, shelter is usually a good idea. Personally, my tank is just straight up water and fish.
i have some pvc cutoffs, clay pots and larger PVC fittings in my ft with yp, bg and tilapia..
i wouldn't put rocks in, as solids can accumulate in them.. and as far as plants, no,, i want all my nutrients going to plants in my growbeds
in my crayfish tanks, i use use sponge filters, and plants (anachris and duckweed) as well as rocks on the bottom of the tank.. they need to be cleaned occasionally - i usually siphon up the gunk between batches of baby crayfish
I've got a few aquarium type decorations in my 100 gallon tank for the fish to swim through, and use as shelter, etc. They seem to enjoy them. Also for me, it's a bit more exciting than a plain tank.