> If there were two scleromochlids known, and the group as a whole
> turned out to be paraphyletic, that would mean one of the
> scleromochlids had turned out not to be a scleromochlid.
If a cladistic definition is in use for Scleromochlidae, yes.
A phylogenetic definition* is not necessary; I'm just saying that everyone
who only recognizes monophyletic taxa ( = clades) above the species level
would in the above case say that one of the scleromochlids had turned out
not to be a scleromochlid.
* Has nothing to do with cladistics, except historically.