i have a 10 gallon sump on my 30 gallon right now, and I understand where fishfreek's coming from as far as small sumps aren't as effective, but I think it depends on what you're trying to do with the sump determines it's effectiveness.
I keep about 8 gallons of water in my sump, which is about 26% of the display tank's capacity - for filtration, I have my overflow line drain into a filterbag that sits inside a filter basket full of floss
I keep my heater, my co2
reactor and ph
probe in there as well, and use a maxijet 1000 as a return pump.
the maxijet also has an extra large strainer on it filled with filter floss, for additional filtration.
I also keep fast growing stem and floating plants in my sump, and use a small light that runs opposite the display tank so I have plants providing biological and chemical filtration 24x7
here you can see a pic of the sump, before I added the plants
I'm not sure what you mean about using a RUGF
in a sump ... it's still highly prone to failure and although you're not disturbing your main tank when it comes time to clean it out, it will still be a lot of hassle to get the sump cleaned and operating again if/when the rugf