I've used lava rock for over 30 years as biological media.
For mechanical filter socks, matts, even pot scrubbies work

When you say skimmer, do mean protein skimmer (fractionation)?
I use fractionation in fresh water tanks, but if the pump doesn't create enough agitation at the air water interface, it won't foam in less dense fresh water (compared to salt).
Here is a video of bio-fractionation on a freshwater koi pond, I had in Milwaukee. I used smaller versions for tanks (its pic)
koi pond fractionation
As for the refugiums, I also use those those in fresh water, but planting them, and stocking with cherry shrimp, gammarus, fish fry and other min-animals.