You absolutely could plumb it into your display tank.
Other equipment, you need to figure out how to get water to and from your frag tank . You will likely use the overflow from the 100 to flow into the 20, then you will need a pump to either return the water to the 100 or to the sump, but if you put the 20 below the 100 and above your sump then you can get away without any pump because you can build an overflow into the 20 to flow into the sump. You will need additional water circulation for the 20 and a lighting system. HTH