Pronoun questions

I've been researching gender-neutral pronouns in the English language this weekend.

I'm wondering what pronouns are most commonly used and in what contexts. I've come into contact with a/ou, though I'm still not sure if I'm using them correctly in a grammatical sense. I did not know that a/ou originated in Old and Middle English.

I'm more familiar with zie/zir and sie/hir, but I'm wondering if they have different connotations or can indicate different things? I haven't found a lot of links about that yet. Most of what I found just seems to explain that they exist and where they fit grammatically.

So if anyone has links, info, opinions, I'd love it if you shared.