If the stabilify was free then i don't see any harm in using it. I wouldnt waste any money on bacteria in a bottle or anything that claims to help your cycle.
If I were you, I would do the water change, test again and see how low the nitrates get. After that I would say test daily and if you see any ammonia then do a smaller pwc
if it goes to .25ppm.
You seem to be in a fish in cycle. Theres some great step by step articles on here that show you exactly how it's gonna go.
That worm might be a bristleworm, I'm terrible at identifying stuff in pictures lol
if it is its your option to keep him or not. I hear they have one nasty sting though. Some people like them so consider them pests. me, i wouldnt tolerate one in my tank if I found one.