What have you been feeding him? I've seen white poo.
Bettas are not the easiest fish to choose tankmates for. The problem is their long, flowing fins. A lot of small fish find these absolutely irresistible and will nibble on them. This stresses out the betta. Often he will respond by attacking them, which will stress the little fish. Bettas have definitely been known to attack, kill and even attempt to eat small fish like tetras.
I agree that peppered corys are too large for that tank, they really need a lot of surface area at the bottom to patrol. Your best option there is pygmy corys. There are three true pygmy cory species - Corydoras pygmaeus, C. hastatus and C. habrosus. My personal preference is C. habrosus because of its pretty markings, but any of them are small, playful and lively.
Mollies I wouldn't recommend, because they need considerably more space, and also their water requirements are very different from the water you've got (and the other fish need.) You might be able to keep platys, if you can find a dwarf strain you could have four or maybe five (with the 3 pygmy corys, betta and 2 ADFs.) They are extremely colourful and easy to keep, but make absolutely sure that the ones you are buying are plump and not skinny. THe ones that are thin and flat bellied have either parasites or TB, and are swimming time bombs.
Tetras and bettas are usually a no no. The smaller and leaner ones are okay in your tank (like rummynoses, neons, embers, glowlights and black neons) but they might go for the betta. I've kept a betta with ember tetras, but I was kind of surprised that tank worked out. I have eight embers and a betta in there.
Bigger tetras need more space and are usually quite nippy, especially if kept in groups of less than ten or fifteen (which clearly wouldn't fit in your tank.)
If you want to keep midwater schoolers with a betta, harlequin rasboras are probably your best option. They are almost never a problem kept with bettas, unless the betta is obnoxious and attacks them (and if yours would, he'd probably have gone for the frog by now.)
You could probably have 3x pygmy cory, 1x betta, 2x platy, 2x ADF and 4x harlequin in that tank (or skip the platys and make it 7x harlequin), with very good maintenance (50% water changes once or twice a week.) Add them over a few months, because this is technically overstocking.
As for the spots... probably best to wait and see, but it's probably the point in the life cycle where it's dormant.