General remark: although no longer extremely triggered by listings of small auchenipterids, I get the feeling now and again that I've made the wrong choice by getting rid of the larger species and keeping the medium ones
i think you're wrong on keeping yourself worried by this
the bigger species would require a complete overhaul of the tank, if you wanted to keep several or least 1 pair of the marmoratus
not at least, because Ageneiosus marmoratus and Tetranematichthys wouldn't co-exist in the same tank!
this because they are probably really bad food competitors, but also:
the marmoratus will need open swimming space as well as shaded open ground space; the Tetranematichthys will need a densely planted tank with medium or large Amazon Swords to rest in.
you cannot have both: an open tank with swimming space and shaded open sandy areas under wood where such a large fish can rest (in a group!) and the dense bushes needed for the other species
so, you still could only keep 1 species.
also: getting rid of all those "driftwoods" wasn't a bad decision either; apart from the fisheri, the other species are gluttons and quite disruptive; they wouldn't be good tankmates for medium/small Agen eiosus at all in the long run.
and if you ever wanted fisheri again, I'm sure you and Bijn could make such a deal that each of you end up with 1 pair.
it's also by far the best looking "Driftwood" and also the most mellow.