1) Are some platforms better or more trustworthy for creating new crypto currencies? Any recommendations?
No,there isn't such thing as 'trustworthy'.Depending on what you have to do with your project,you can choose to Fork an existing project,clone an existing project and make changes in their algos or use frameworks like Lisk/Waves/Ethereum to create smart contracts.This also depends on the developers you will find for the team.
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?
You need to study a lot more basics about cryptos in general,my comment here wouldn't help you understand all of it.
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?
Do you understand protocols ? What is your project ? It is too early to advise you on anything.Please read more about stuff.
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?
Thanks!
Nope.Only thing I can recommend is,use google to know everything that's been asked for.