I think it's more appropriate or respectful to use ます with strangers at first. However, if you make a mutual decision to talk in くだけた日本語/informal friendly Japanese, then by all means that's the route you should go. I think if the person talks informally from the get-go and is your peer or younger than you, then match their tone.

I think, it's on you. I use the polite form in writing entries. It's better if we use polite form since we barely know people here. (That's just my opinion though.) You can use informal form, if you want, of course.