Well, your temp should be stable through heaters and fans... or at least (if your lighting isn't producing too much heat) you can probably do it without fans. I think a half-degree swing is a little too much to acheive without a controller of some sort... I had my tank swing one degree or so through 24 hr periods, and did this for quite some time, with no ill effects.
And yes, your pH should be stable too. If you are constantly having to buffer it to keep it up, you have another underlying water chemistry issue. And, FWIW it is far better to have low pH (say 7.8 or so) and leave it steady than it is to buffer it, let it drop, buffer it, repeat... that is rough on livestock.
As for fish choice, yes those fish are too big for that size tank, so they will be under even more stress than any other new addition would normally be. Stress is a big factor in illness and fish loss, especially the PBT which could be one of the poster children for a stress-prone fish.
As for "nursed them back to health", what I mean is I fed carefully, made absolutely sure that my tank environment didn't contribute to further problems... I didn't QT, because pH shock can't be medicated for, and QTing won't help. All you can do is make them as comfortable as possible through careful attention to the tank conditions.
Also, you shouldn't have to add stress coat when acclimating a new fish either. I don't know that it could create any problems, and maybe others will chime in as to whether it can benefit a fish that is showing distress... but under normal conditions all you need to do is choose your livestock wisely, acclimate the fish correctly... and yes, you should quarantine new fish even if many don't. Some people don't, and haven't for years, and have never been bitten in the butt by it... but even they will admit that it could happen. IMO there is no reason to take a chance when all you lose by QTing a fish is a few weeks of not getting to see them in the display. Another reason to QT is that if one does show illness, it can be horribly difficult to try to catch them in a stocked tank... if you can verify health before they go in you can avoid that nightmarish job... not to mention that your other fish can't become infected.
We aren't trying to come down on you; we all had to learn this stuff once. The only way you'll get any flack around here is if you ask for advice, and then do the exact opposite.
Again, not trying to come down on you, but you do understand why I said I was going to jump in and be the first to scold you for cycling with the damsels, right? Were you advised to do it that way by a fish store?