https://journals.flvc.org/fshs/article/download/86329/83245 OBSERVATIONS ON THE COMPATIBILITY, GROWTH, AND CROPPING OF CALAMONDIN, ‘MEIWA’ AND ‘NAGAMI’ KUMQUAT ON SEVERAL ROOTSTOCKS
This is a paper which includes some details on graft compatibility of kumquats as scion. I really recommend using Google Scholar to look for compatibility matrices. There's a lot of low-hanging publication fruit out there if someone wrote some natural language processing bots to scrape new publications of compatibility data and provide meta-analyses on a regular basis. Heck, even just a hand-edited website which combines published data and makes it available in an easy-to-digest form would be lovely.
https://ucanr.edu/sites/ucceventura/files/57694.pdf More generally, this is a good at-a-glance reference which I think matches the the Citrus Production Manual, but I don't know what edition, and I'm too lazy to go find my copy to confirm that memory. I dunno, you can see why Japanese citrus horticultural tradition is to always use Poncirus as rootstock: great compatibility and results with everything but lemons! (And I think there's options to use interstems if you do want to grow lemon on Poncirus, but that's not something I bother to remember.)
Also worth noting that some graft problems are common, but survive long enough for industry purposes, though the commonest set of those I'm familiar with is with mandarins as the scion. You get 15 years out of a tree, and then you're done. So nurseries may be selling grafted trees with industry-standard combinations which don't match the expectations of home growers, but do match good horticultural practice. In other cases, simple ignorance rules, where small producers simply don't know that compatibility issues are a concern, and just graft everything on C-35 because that's what they think is a "good" rootstock.