If you are going to buy some Synodontis nigriventris, it is well worth doing the "in for a penny, in for a pound approach" just like with Kuhli Loaches, as in forget about minimum groups of six and go for minimum of ten. They are very social, just liike Corydoras, but also rather shy. The only time my nine come out into the open is after I've gone to bed, after getting a night vision video clip of them.
Have you got room for another 9/10, ~7cm fish? Borderline I reckon, will need big weekly water changes to keep nitrates under control.