Adding To Established Shoals

Hypothetical situation but one which is floating around my head as a possibility, so I'd appreciate your thoughts on it.

Tank is 54 litres but, taking into account the slightly lower waterline (minimises the risk of fish jumping out) and decor, is probably more like 45 litres of water.  It has 2 filters in it, and pre- water change readings are consistently 0 ammonia, 0 nitrite, 20/25 nitrates.

Currently 5 x-ray tetras (2 females, 3 males) and 6 harlequins (can't make up my mind on the sexing - the shape of the wedge doesn't seem to tally up with the body shape but there are 4 large and 2 smaller ones).

As you know, I've toyed back and forth about whether to fit more fish in (but this doesn't seem do-able without overstocking) or whether to wait until nature takes its course and then get a new species (cherry barbs or cardinal tetras being the most likely - I'd err on the former; Mr FCMF prefers the latter).

Another option which has been playing on my mind is whether I should increase the shoal sizes, in particular to add more female x-ray tetras to calm the males down from chasing the females and to give the females some breathing space (and the injured one an opportunity to heal). If so, my options would be:
* add 2 female tetras (one on its own in a QT for a month would probably stress it out, and I wouldn't do any less than a month's quarantine after Sue's experience with import whitespot and Fiona's experience with whitespot setting in near the 3-week point) - 85% stocking according to the CC
* add 2 female tetras and 2 harlequins (as the harlequins might feel better in larger numbers .v. the boisterous x-ray tetras who tend to take over the tank with their chasing antics in the mornings; also, the small harlequins might give the 2 existing small harlequins some similar sized company, and presumably one on its own with 2 tetras in the QT wouldn't be a wise idea) - 96% stocking according to the CC

Thoughts welcome on each of the above two possibilities - or indeed anything else which I may not have thought of; thanks.

Thanks.

Has anyone ever had problems with youngsters joining established shoals which have only had one another for company for a couple of years and with no new additions?

I missed this thread @fcmf 

It depends on the fish. Those species that are known to be nippy because they have a hierarchy within the shoal would probably pick on any new fish, particularly if the new ones were too small to fight back.

But fish like Lellynelly's harlequins and neons are not like this, and it is quite safe to add more to those already in the tank.

Another forum I use doesn't like aqadvisor. Before they moved to a new host, the very name of the site was treated the same as swear words and removed from posts. They reckon it allows some very iffy stocking.

Mind you, they don't like the community creator on here either, though the CC doesn't get mentioned nearly as much.