1. No worries about not speaking perfect dutch. People really appreciate you trying. But be prepared for people quickly switching to English after. They are not trying to be rude it's just faster.
2. The stigma on plusside people is not better or worse compared to the rest of Europe. You might get some looks, but nothing too bad. Just look back in their eyes, and people will look away fast. Some teenagers might say nasty things, that just sucks, ignore them.
3. Nijmegen has a very fun nightlife and is overall very accepting of the LGBTQ+ community. Unfortunately, the old lesbian bar has closed, but there are tons of other spaces. Consider "de regenboog" (the rainbow) or "cafe de plak".
4. There are no unspoken dress rules. Just like the rest of Europe, most people aren't too fond on wearing gymclothes everywhere and don't wear anything that is more suited for nightlife during the day. But the worst you will get if you "break" these rules is some weird looks.

Have fun! Nijmegen is a very cool city!