Sykes

There is no secret stocking formula because there's so many factors that go into it: type of fish, plants, what you feed, how much you feed, water change schedule, filtration, and the list goes on. It'd be impossible to capture all that in a formula, so I think the inch per gallon rule caught on because it's easy to remember, easy to apply, and gives a reasonable estimate. As you gain more experience, you can fine tune what works for you. Personally I think your stocking level is pretty reasonable, but I might just be giddy to see a betta going into something bigger than 0.25 gallons (poor things!). Are those plants in your tank live or fake? If they're live, that'll also help keep levels in check, and I see you have a test kit standing at the ready so that's a good sign as well. That said, I don't have any experience keeping bettas with other tankmates, so I'd defer to others on compatibility of those 2 species.

A good thing to think about here might be population management. Platys, zebra danios, and guppies can all be prolific breeders. If you have plants or other good hiding places, there's a good chance you'll start seeing babies soon even if you don't intend to, and they can quickly overrun a 20 gallon (I've had zebra danios alone overpopulate a 29 gallon, ended up donating well over 100 offspring to my LFS). Do you have a plan for what you want to do with the babies? The answer may restrict what else you can include in the 60 when you upgrade.

Likewise, I have 2 tanks with both bristlenose and swords in them and both continue to look healthy. I have heard of them being devastating for some folks though. Maybe give it a try but have a backup plan (i.e. another tank you can move the plecos to) in case you start seeing thinning on the leaves? I don't believe they actually eat the leaves, but the leaves can get damaged as they attach to it and eat whatever's growing on the leaves.

Definitely lots of good ideas here already... a couple more for the list, though personally I'd go for CPDs: Green Kubotai Rasbora Otocinclus Catfish

I'm breeding Pineapple Swordtails currently and from both what I've read and observed, if a female has been in contact with a male and is of breeding age, she's almost always expecting. I'm sure there are pros out there that could tell easily, but I always have a hard time knowing exactly when she's about to pop. My approach has been to provide tons of plant cover (preferably live plants but some fake options work well in a pinch) and just let them be. Next thing you know you'll start finding fry swimming among the plants.

I'm assuming the shirt was placed in a hang on back filter? If you haven't I'd recommend removing it and giving it a quick scrub down (no soap) since that's where the you'd have the highest concentration of toxins if there are any. You can keep your sponge, or whatever filter media you have, in a small container of tank water to make sure you don't lose any beneficial bacteria, although for a short time I wouldn't be too worried about that. I don't typically use carbon, especially for planted tanks, but removing toxins like this is a good short-term use for it as Brandy suggested. Beyond that, just daily large water changes until you get it cleared up. I expect you'll see improvement pretty quickly, but if you don't I'd double check to make sure there's nothing else in the tank that could be leaching chemicals. Good luck, and kudos for planning ahead well enough to discover problems like this before you put fish in!

@Brandy, I'm jealous, that piece looks beautiful! @Patrick M. Bodega Aquatics, generally agree with the other posters here. I have maybe 30 lbs of driftwood in a 75 g. I boiled them in the largest pot I could find 3-4 times for an hour+ each. Some of the pieces I had to flip and twist a few times because the whole piece wouldn't fit in at once. The pot water was never 100% clear after boiling, but it'd get slightly clearer each time. Purigen took care of the rest so the tank water never darkened. Haven't got my hands on a piece too big to boil yet, but I'd guess that if you're able to get some water flow that might help speed up the process in lieu of heat.

I've done both methods in the past and never had a problem with either. If I'm using 100% tap water, I'll typically just dose Prime to the full tank volume and add directly to the tank first. For added caution, I typically pour it in roughly the same spot that I'll be adding the new water (via Python) so that the current of the new water forces it to mix. I've done 50%+ water changes this way on a 125 g without issue, although I've since upped my plant game so large changes like that aren't necessary any more.