I would say no to the ADFs too, you would need a pair of them, and their bioload is quite high since they're messy eaters. They also might get nipped by the Neon Tetras because Neons are quite nippy. ): Really I would say you're at your max right now, I would up the number of Neon Tetras, depending on how many you have, if you have 6 maybe go for 2 more or 4? That might be pushing it, but if you have a good filter and live plants that'd be no problem. 1 Betta and 10 Neons.
Also for the ADF in the 1.5g no, again. I would say 2.5g for one is good but, once again they're social beings. So they need to be in a pair.
The test kit you listed isn't really kit, it's only for ammonia. But yeah, getting a liquid test for ammonia is always good, but I would get the whole set, with nitrItes and nitrAtes too, because testing those are important too.