If you have aeration and surface turbulence then you can cover about 90% of the tank and the airstones bubbling away will provide sufficient oxygen/ carbon dioxide exchange. However, having 90% coverage makes it hard to feed the fish, especially if using dry foods so most people go for 50-75% coverage.
You can make plastic loops from airline and a joiner to stop the plants spreading across the surface too much. Just grab some airline and a joiner to make a loop. Put it around the floating plants.