Shoal size/tank capacity

[Mr44]

Hello folks, new member and C. sterbai keeper here. I have 6 of them in a 15 gallon (24x12x12") tank planted with Anubias and Java Fern, along with 1x small Ancistrus sp (?) and 8x Harlequin Rasboras. Water quality is excellent, with Ammonia and Nitrites at 0ppm and Nitrates steady at ~5ppm or thereabouts. Filtration is 1 Aquaclear 150 and an extra AC Mini that I had on hand, so lots of current and easily 10-15x per hour circulation.

I am tempted to re-home some or all of the Rasboras in a different tank and add up to 2 more sterbai, since I can't help but wonder if they'd be happier and more active if they were a bigger group. As it stands now they will all come out for food, but otherwise if my tank light is on I may see only 2-3 cruising around if I peer into the room without them seeing me, while the rest hole up under various flowerpot pieces or driftwood. Would this be a wise course of action? Would I even need to move the rasboras?

[apistomaster]

Having more of any Corydoras is usually a good thing but I don't think this will change their behavior much. My sterbai have always been on the shy side. They stay under the shelter of plants or wood and mostly feed out in the open when they are offered live or frozen foods but then retire back into the shadows. They are at their most active in twilight conditions. I see them overcome this when I am raising them and may have 100 in a grow out tank to themselves and juveniles are more adventurous or less cautious than the older breeders. This has been my experience.
They don't hide 100% of the time but they never really are out and about all the time. True schooling behavior seems most common when they are in the very large numbers of 100 or so and even then, most often when they become frightened. They otherwise will sit as rest as a group under their favorite shelters most of the time.