Can I use tea bags as mulch to fixate some nitrogen?

Ask yourself "what am I trying to achieve here?" first before u do anything. Why do u want to add these things and what made u think they will do anything let alone anything good?? Tea bags to fixate nitrogen?? How? by what means will this do anything aside from slow water evaporation from the medium? It will take months to break down into plant food- weeks past the time u have dried and started smoking your plant. So why add it? Now to the root stimulator to boost bacteria. They are not a bad idea, but not a great idea either. in hydro and coco they are great. In DWC they are magic. in soil they are pointless. if u have organic matter in your soil then its full of microbes. more than enough for a grow. Something like Hy-Gen Humiboosta would be a much better option at this point rather than some additional microbes or mulch. Deep sea kelp/sea weed extract used as a tonic is never a bad idea either. Dont go overboard on it as some can be high in things u dont need alot of. But its good stuff too.

Teabags no, seaweed tonic yes.

Yeah why not! I’m a huge proponent of beneficial bacterial, myco, etc. I swear by Microbial Life Hydro Photosynthesis plus—- add SLF-100 and teas with sweetener to feed the bacterial. Good luck