I couldn't find an answer to this after searching this form.

When doing the Small Universe meditation is one supposed to use reverse breathing (stomach in on inhale) or just relaxed natural breathing (stomach out on inhale).

Thank you in advance and this forum is awesome.

Generally, in SFQ circles, we try to avoid the term "reverse breathing" (though I know what you mean), as it adds unnecessary complication.

We just call it "energy breathing" (lower stomach, just very slightly in on inhale, so upper stomach and chest will expand, and then just release on exhale. Entire movement is very slight and also should be fairly relaxed. Focus on skin breathing in and out). As far as I understand, in ALL SFQ practices, we use "energy breathing" as best. However, there is always, "good" and "better" still available for use!