So far, teh only paper (which we've discussed before on RC) I've found looking specifically at how skimmers alter microbiomes in reef aquaria is Feldman, ea al, paper
Bacterial counts in Reef Aquaria which showed skimmers do significantly alter microbiomes. "Thus, skimming inadvertently provides severe (?) evolutionary pressure to skew the tank’s resident water column bacteria population to favor the “non-skimmable” cohort." SOme may consider this a bias on my part but inadvertantly altering something so essential as the microbiomes in a system without being able to quantify what is happening seems to me something to be avoided.
But you're right, We don't have any research specifically showing skimmers directly promote pathogenic OR beneficial microbiomes. There is research like
this paper linking reduced diversity with compromised immune system in corals. We also have a lot of research in other fields showing disrupting microbiomes has severe ramifications. In extreme killing the host organism or ecosystem. Notable examples are the thousands of people dying every year after antibiotics disrupt their microbiomes which lets Clostridioides difficile
(1) (2) take over and the world wide concern over
unsustanable farming practices literally sterilizing farmland.
As far as what I've observed I'd ahve to agree with the concern expressed by the researcher in
this paper "Although captive breeding and propagation of corals is a well-known activity among aquarium hobbyists and public aquariums, the link between coral science and aquaculture is still poorly developed". Reef aquarists are being advised to do things that are not supported by the available research or even contraindicated.
I certainly agree, there's lots of things we do that impact the various microorganims in our systems. But the organisms and microbiomes in our systems are impacting each other and we can't test for these processes. Sadly we very much need research giving us a better understanding of what is happening and how to quantify these processes. Until then ignoring the warning signs given by resaerch in other fields doesn't seem wise to me.