It does mention group size in the profiles, either under the Compatibility/Temperament section or under Discussion. If you find one that doesn't, let me know and I'll add it.
For Carnegiella species of hatchetfish, more is always better. In your 3-foot tank (surface area is the prime concern) I would say around 12+ Marble Hatchetfish. So another 8-10 added to the 4 remaining. When i had my C. strigata in my 3-foot I had 21, which was cozy, but they are now back in the 115g but they congregate on the far right side regardless, within about a square foot, at the opposite end to the filter outflow: they do not like currents.
For pencilfish, more again is always better. With the species in question, I have had very good luck with Nannostomus mortenthaleri as a group of 7 in my 3-foot 33g. Originally I had 8, 4 each m/f, but one died for reasons unknown. This group is settled now for over a year, have regularly spawned, in fact I have two surviving fry now over half an ich and beginning to get their colouration. So that ups the group to 9.
But 7-9 is my suggestion for that tank. I believe you have lots of plants and wood; these fish absolutely need both, they do not need swimming room, but they do need cover, above and within.
The N. rubrocaudatus, if you go with them, same as above. Wood branches are even more critical, as this fish occurs only in a watercourse in Peru that is thick with wood and branches, and the fish remain solitary, each around a branch, which is why they are so expensive: collecting them one by one in a tangle of branches is not easy.
This is probably my least favourite of the genus; I never see them.
If you have the true dwarf pencil, Nannostomus marginatus, a larger group. I would get 11-12 in a 3-foot tank.
Nannostomus eques and Nannostomus unifasciatus are almost identical--the latter is very rare, I've only once seen it locally. These also need a decent group. No less than 12 of whichever, or a mix of both is nice, 6-7 of each species. I currently have 12 N. eques and they regularly spawn but the tank they are in (my 70g flooded Amazon forest) has too many inquisitive fish and the eggs get eaten. I have my N. marginatus in with these, they never interact or bother each other, very different behaviours. The N. eques and N. unifasciatus are interesting for their angle at 45 degrees. They tend to like to stay close for the most part. Water Sprite is an ideal floating plant with these, they spend all day among the dangling roots, looking for microscopic plankton.
I see no problems with the numbers I've suggested, all together in the 3-foot tank. I would not add any more cichlids though; my A. baenschi found the N. marginatus a threat, and constantly went after them when the female had eggs and fry.