O.O sorry to interrupt.
Are you feeding your fish just pellets? Because if they haven't grown at all, you might try adding a frozen food (either brine shrimp or glassworms) to their diet two or three times a week. It's possible your fish are just small, and the best way to find out really is to add some good ol' fashioned as-close-to-live-as-possible protein to their diet. :) I know that the NLS pellets are AMAZING (totally my favorite pellets for my bettas) but frozen food is the best as far as nutrients, and the closest you can get to a betta's natural diet without going live. And you're right about you being the best to tell if you're feeding them the right amount, because you get to see them every day. :)
And as far as your stringies, it may or may not be a fungus. If you can put him in a nice comfy hospital tank (a 1 gallon if you have one, with a filter, heater, and a light) with a proper dose of AQ salt, and some stress coat for a couple days, you should be able to tell 100% either way (because it'll either get worse -in which case its a fungus- or go away and get better). The best thing I've learned to do if a betta is showing signs of any kind of sickness is to QT him with fresh water and the AQ salt and keep an eye out, because until you know for sure, you don't want to medicate, because you don't want to incorrectly medicate him and/or weaken his immune system more.