1) Are some platforms better or more trustworthy for creating new crypto currencies? Any recommendations?
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?
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?
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?
1. There are advantages and disadvantages for the major different platforms. ERC20 has become the most popular platform for marketplaces because it includes smart contract features that make it really easy to automate payment mechanisms. WAVES is one of the easiest platforms to develop and launch a token. Look at the lists on CoinMarketCap and study projects that are good templates for your idea.
2. Mining is a process for creating new tokens, usually as a payment incentive for people that contribute infrastructure to support your project needs. If you really need a fixed supply, then don't publish mining details. Usually there is nothing wrong with some level of "inflation" in your token supply, as long as it is relatively low and reflects added value for users at a faster rate than the increased supply of tokens to support the growing infrastructure in your project ecosystem.
3. Do more studying. Yes, it is really easy to distribute tokens to an ETH wallet, but that should be way down your list of priorities. Focus on the attributes of your customers and key partners and the functions you need your service ecosystem to perform. Form follows function at all times.
4. Exchanges don't care what your platform is. They care about a pool of investors that are actively buying and selling tokens; making a project successful should be your priority - your investors will bring your tokens to the exchanges they prefer, and you can always act as your exchange and make a market for your token by taking ETH or BTC as payment to get onto your service.