The sump refugium combo is pretty much just for the avid reef tank keeper who wants an area he can put live rock in and supply light to when the main display if off. This allows the Macroalgea that you growing in the fuge to filter the chemicals in the water so that it makes it harder for nuicance microalgea to get a foothold, and it also makes a great area for pods
to grow and feed the main tank. If you do reef then sump/fuge combo's are great but for just a fowlr
it may be a bit overkill. The reef site just lets you know how to go about designing an acrylic sump from scratch. The design and size are all up to you.
And yes the sumps that are made from smaller aquariums are great asswell and very affordable. I've made 3 of them myself for a 75 gal
cichlid tank, 20 gal
nanareef, and a 210 gal
cichlid tank. The one for the 210 was an old 30gal tank that I siliconed sheets of glass in for the deviders and plumbed with a magdrive 1800. I never had a problem with out of wack water parameters. That was with a severly overpopulated amount of cichlids.