An Emperor 400 would be a great choice. I wouldn't modify the output path. The filter is designed to give you maximum surface movement producing increased oxygen levels. If you cut down the surface movement or redirect the flow you'll decrease the oxygen level. If you wish to adjust the output flow rate there's a grey button-like device within the filter. It needs to be all the way up to get the filter flowing properly. After the syphon has started, and it's running right, you can push the button down. Each click cuts about 25% of the flow rate. As your fantails grow their anal opening will produce larger waste products. You'll need the stronger flow eventually to keep the tank clean.
BTW, a 20H (high) is normally 12X18X24, a 20T (tall) is a double 10G 20X10X20 and a 20L is 30X12X12.