Are the adhesive properties of greenstuff and milliput strong enough that superglue is not required for them when filling the depressions in mantic bases?
I’ve never worked with milliput or greenstuff.

Long while ago I talked of using oathmark skeletons on 20mm square bases.
I was luckily lied to when they would seem only usable on multi basing.
If you trim their puddle bases they will fit on the 20mm square bases just fine. Even the mantic ones with the lower circle side facing up as those bases have a little bevel.