2) I want this currency to have a fixed circulating supply. Does this mean that my coin should not be mineable? (I don't really know how mining works). Any recommendations?
Tokens developed on any platform are non-mineable. I think that answers your question - you are developing a crypto "on a platform" so thats not mineable. However I am not really sure if this is 100% correct.
I believe I have seen mineable ERC20 tokens on the Ethereum blockchain.
Yes, I have, here's an article I remember reading:
https://lightrains.com/blogs/how-to-create-mineable-erc20-tokensI don't know about mineability on other chains. Seems like there would be major downsides to a minable token that has a dependency on an underlying currency.
Best regards,
Ben