In connection with my intense fish broodiness last weekend, many thanks for the responses to the poll. The decision I ultimately settled on was ember tetras. For various reasons, I wouldn't be acting upon this until the new year.
I wasn't convinced I had enough algae to support otocinclus / a BN plec and I was also a bit concerned about how it work with them in the QT for several weeks eg I could move some of the plants into that tank for them to graze upon/clean up but I'd then feel uneasy about moving the plants back into the main tank before being sure that the newbies were disease-free. So that really left me with the microdevario kubotai .v. ember tetras, and the microdevario in the shop were quite large already (definitely 3cm as some others on the internet seem to mention is often the case) and very fast-moving, so I felt embers were a better option stocking-wise - and would complement the colours of the existing fish, a contrast to the green plants.
Now that the intense broodiness has subsided (wouldn't take much to re-fuel it, so avoiding going near the LFS!) and I'm back to my usual self, I'm wondering how wise this idea is. I'd absolutely love to have them but I'm unsure how my middle-aged (3 and 2.5 year-old shoals) fish would adapt to a shoal of newcomers in such a confined space, and whether increasing the stocking capacity at this stage is wise given how health is likely to decline in the latter half of their lives. The 3 male tetras are 4cm each, the 2 female tetras are 5cm and quite a lot larger all round; 4 of the harlequins are 4cm each and 2 of them are 3cm each, so that puts some of them larger than the CC's estimates of 4cm and 3cm respectively. I'm also a little concerned at whether the newbies would find some of the larger fish quite intimidating - and therefore whether I ought to wait until nature takes its course, after which I opt for embers and similar-sized fish. Am I being overly cautious or sensible in my concerns? Views welcome; thanks.