With that fish list, I'd say you definitely need to start over with fish anyway. I wouldn't start over with the cycling as you definitely have good bacteria already.
There are a couple problems with your tankmates. I saw in another post that you have a 17 gallon tank. If those are common plecos, they need a 55 gallon tank minimum. Also, tetras are schooling fish so you need a minimum of 3 and preferable 6 of the red-eyes. From looking at a profile on the ballon mollies, I see that they need a 30 gallon minimum with lots of plants. Mollies also prefer brackish (salty) water so they probably aren't good tankmates for your other fish. I'm not real familiar with the danios but would think they need schools too.
I would definitely return the plecos (they get very big and are also big waste producers), and mollies (brackish water). I would go with a small school (6 or 8) of the red eye tetras and maybe a bristlenose pleco or bristlenose catfish for your algae eater. You should be fine keeping the angel FOR NOW but angels will get to about 6" long but more importantly, need a deep tank as they can get 12" tall or more with their fins.
As for testing after a water change, I would give the water about 10 minutes to mix/filter and then check it.
