Here are some links to help you pick your instance (give you an idea of what behaviour is expected, if there are any subject specialisations, etc)
For using multiple accounts I like Subway Tooter. It's open-source, but not on F-Droid. I think it's mostly by Japanese developers, so you should considering submitting translations if you notice any oddities. I've done it before via Github and it was easy if you're familiar with the web interface.
MastoLab is another popular, multi-account one (open-source and on F-Droid), but I had trouble using multiple accounts that shared a domain.