Yes, you can use Squirrly SEO alongside Rank Math if you want to, as Squirrly enables you to deactivate certain settings or modules so that Squirrly SEO and Rank Math can work in compatibility mode.
If Squirrly stops handling source code changes, then it works with RankMath, SEOpress, AIO SEO, Yoast, and others.
Regarding whether you could replace Rank Math altogether with Squirrly SEO, the graphic linked below shows you a side-by-side comparison of Squirrly SEO vs. Rank Math.
^^ As you will see from that graphic, Rank Math’s Ranking section does not use a professional SERP Checker cloud. They only give average data which they take via Google Search Console.
There are 650 features in Squirrly SEO, most of which can’t be found in RankMath, given that RankMath does not offer a duality between plugin and cloud services.
You can also learn more about the features Squirrly SEO provides here. You can search on that page to see if Squirrly has a certain feature that you are interested in.
Hope these resources will help you decide whether you could replace Rank Math with Squirrly SEO altogether.