No way will a 55 gal barrel work here. Way to small. Not enough dwell time. Bottom drains remove heavier than water waste. The settlement filter or Vortex is a very large wide spot in the line that slows the flow down enough to allow the heavier than water solids to settle out. I like to use one gallon of volume for every gallon of flow. Some basic's 1st
Your pond needs two 4 inch bottom drains.
Each bottom drain has to have a min. of 2500 gph flow to sweep the lines.
Two 4 inch bottom drains @ 2500 gph min. would be 5000 gph flow, = a 4 to 500 gal settlement filter or vortex.
For the ponds I build for people I use a 500 gal cone top AG tank i turn upside down and modify into a vortex. (see picture)
From the settlement filter or vortex I gravity flow the water through some kind of Bio Filters, then the pump, then back to the pond via 3 TPRs like this. (picture)
Lets stay with the basic pond size and shape, bottom drain circuits 1st, then we can move on to the skimmer circuit.
The size of your pond is good, go 6 to 7 feet deep if you can, make the corners rounded, so the pond is like an oval more than a rectangle.