I pretty much concur with what fishorama said. I am even more against the stress coat and easy balance than she was.
Your parameters look OK for clown plecos. They look less good for the mollies (females can be 5+ inches) which would prefer harder water than you have. Yours is on the soft side which is fine for the tetras and plecos had they lived. A better option for the yoyo loaches (which grow to 6 inches and should be in groups) would be Ambastaia sidthimunki aka dwarf chain loach they are active all over a tank. I keep these and some of them have been with me for 18+ years.
https://www.loaches.com/species-index/yasuhikotakia-sidthimunki
There are two ways that fish are measured. TL= total length and means from tip of the "nose" to the end of the tail. SL= standard length which measures from the tip of the "nose" to the end of the body and does not included the tail. The scientist prefer SL and we hobbyists tend to prefer TL.
Based on your answers I still lean towards the plecos not being all that healthy to start and I think the combination of a longish acclimation and the idea that your water was not in the best of shape due to water changes being less than idea.
I am a bit nuts when it comes to researching the science behind this hobby. I read a lot of scientific papers. When studies of fish we are done in a lab, the fish are usually acclimated first. The time involved is 2 to 4 weeks. Experiments dealing with the ability of fish adapting to different parameters tend to show some of the physiological changes do not occur for a week or even two after the fish has been put into the new water. The upshot of this is if a fish truly needs to adapt to new parameters, this wont happen in a matter of minute or hours or even a few days.
Finally. most fish and pet stores use a common water system between groups of or even all the tanks in a shop. they will share nets and specimen boxes between the tanks as well. So when a disease or parasite hits, it can affect the fish in multiple tanks. This is why it is so important to use quarantine tanks for new fish. But, this also likely means the problem with the clown plecos was specific to them and not to the tank or group of tanks.
It is important when buying fish from shops that you check everything out. Walk around the place and look into tanks. Se how things look. Are there dead fish? Are there fish swimming weirdly? Do the fish have decent colors and do they appear to be well fed? If you can do so, do not buy on your first visit. Talk with the people who work there and ask a few Qs. See if they appear to know what they are talking about and are willing to help a new fish keeper. If you are not sure about what you are told, you can post it here and ask for opinions.
Come back to the store in a week or 10 days and look again. Does the place look OK? Are there a lot of the same fish still there? Do they look better or worse than on your first visit?
You have alot of options when it comes to buying fish because of their availability online. Finding sources which have healthy stock is a must. Price matters, but buying quality healthy fish even if more expensive up front will be cheaper in the long run. (I always say that one live healthy fish shipped to me is worth at least 3 DOAs.) Finding a local fish club is also a great help. The combined knowledge and the number of members breeding some of their fish and selling them at meetings can be a great source fish and plants often for bargain prices.