I guess I have a few questions...
1) Are some platforms better or more trustworthy for creating new crypto currencies? Any recommendations?
For currencies based on other blockchains that allow developement of Dapps, we have ethereum, waves and Omni platforms. There are also many other platforms but less popular. If you want to go with the safe side my suggestion will be the ethereum blockchain to develop a new cryptocurrency. Waves is also good but it is often not given the "first preference" - also its not considered to be a legitimate platform (do your own research).
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.
3) My coin will mainly function as a currency. I was going to use the ETH20 protocol because hardware wallets seem to easily support it. Would there be any advantage to using a different protocol?
You simply answered your first question. It is ERC-20 token. Its a popular platform. But like I said above there are other platforms. Do check the out.
4) I'd like to eventually get listed on Bittrex, Poloniex, etc. Does this mean I need to avoid certain coin creation platforms? Any recommendations?
No. Your coin will be considered as an asset and should be trade-able once listed. For listing you will need to pay fees. That has nothing to do with the platform.