I’ve never done fractional shares even though I know that some of the biggest brokers offer them. You’d have to ask them if Ameritrade has that. I’m not sure.
To better answer a previous question you had…you’d want to use GTC with limit orders, yes. That means “good til canceled”. Each broker has a different time length that at GTC order stays on their books. For some its 60 days, for others its 90 days, etc.