Sorry for such a belated reply/update but thanks so much for your detailed response,
@Matt - very much appreciated and indeed very helpful.
I've since been having further tinkers with the tank and went back to the original filter outflow pipe. The spray bar seemed to segregate one or two harlies from the rest of the tankmates and they sat facing the walls which meant I couldn't see them at all, as well as the other downsides I mentioned. Then, when I replaced the original filter outflow pipe, somehow I managed to alter the flow in such a way that it seems better for all concerned i.e. possibly creating more surface movement and less sub-surface movement. I also realise that I must have been overly worrying / paying credence to its role in the slight S shape of the harlies' spines which actually would scarcely be noticeable to anyone else - Mr FCMF hadn't noticed it until I pointed it out from above and even then he didn't notice it straight away. What is particularly interesting is that the cardinal tetras, on entry to the tank today, have been spending most of their time in/around that area of concern and aren't getting buffeted at all, so I was worrying completely unncessarily about how they would fare - it's as though the neon green rasboras are choosing to position themselves at such an angle in order to enjoy getting buffeted.
As for the floating plants, I actually keep them in fish feeding rings like these
https://www.ebay.co.uk/i/193112867241 in order to stop them zooming round the tank and ending up in the filter inlets. This probably isn't the best for them long-term but at least it means their leaves don't get too wet.
Separately, over the past week or so (i.e. during the 4th/final week of quarantine), I'd been gradually transferring some tank contents from the main tank to the quarantine tank, and swapping water between each tank to get occupants used to one another's chemicals (e.g. pheromones, etc). I'm not / wasn't sure how much logic there is/was in this, but, given at least one fatality within 24 hours of transfer from QT to main tank among the neon green rasboras last year, I wanted to minimise stress for both parties and maximise smoothness of transition by maintaining similar environments. What was particularly interesting was the behaviour of the main tank occupants on the two occasions when I put a jugful of tank water from the QT into the main tank - the two neon green rasboras seemingly inexplicably started behaving most oddly and separately from one another... one sunk into a silk plant and kept flailing itself, while the other alternated between darting around and shuddering behind a plant. Meanwhile, the harlies huddled up together, while the x-rays continued their usual behaviour. It was almost as though they sensed the presence of additional/new fish but couldn't handle being unable to find them. There was no particular odd behaviour in the the QT although I had put some of the main tank water in it from the outset.
Today, the transition went more smoothly than expected - a first for me! I managed to transfer the filter media from the QT filter into the main tank filter, then, with the filter and decor removed, seamlessly guided all 6 cardinals into a jug and covered it with a towel, talking reassuringly to the fish as I could. I kept the filter off in the main tank so that the new occupants could be gradually emptied in without getting buffeted by the filter outflow. They entered together, shoaled up at one end of the tank (about 1/3 of the distance from the bottom) but not too tightly, while the others were in the diagonally opposite side (about 1/3 of the distance from the top) - it was almost as though a sports match were about to start. After about 15 seconds, there was a mix as they all intermingled, then they resumed their normal positions - harlies near the top RHS, neon green rasboras swapping between filter outflow and intermingling with the x-rays (this started very recently following the demise of two of the remaining four harlies), and two of the three x-rays having spats with one another. All 6 cardinals have been loosely shoaling at the front of the tank, also in the filter outflow but not being buffeted by it at all, and occasionally a couple of them separate to investigate elsewhere on their own. Feeding went well, so my concerns about two of the six cardinals being quite small are unfounded as they seem to be keen to pick up fallen pellets. To my complete surprise, they have been completely unfazed by the tank light from the outset - I'd meant to turn this off but completely forgot - despite only having a dimly lit table lamp near the QT. Fingers crossed that this good start continues!
Edited to add: By the time I finished this post, the cardinals had dispersed and everyone was intermingling. To my surprise, especially as the cardinals shoaled up together at night in the QT, everyone seems to have dispersed. Only the two snails are cutely cuddled up together in the top LHS of the tank which has become their new favoured sleeping location.
Edited again to add video: https:///youtu.be/gw3OFb1qIhI [remove 3rd / from near start of url to initiate it]