Hi @tmora ,
If you are going to use Slim SEO Schema with Yoast SEO, then it’s totally fine. Slim SEO Schema (and also Slim SEO Link Manager) works well with all SEO plugins.
However, we don’t recommend using Slim SEO free with Yoast SEO. Both plugins offer similar functionality and there are a lot of overlapped things. Slim SEO also does some things automatically, and that might conflict with Yoast SEO.