"This shirt is a little thin."
The impression I have is that うすい is used for "thin" meaning flat, and ほそい is used for "thin" meaning narrow, like a pencil. If it's "thin" in a way that means you can wrap your hand around it more easily, it's ほそい.
Also, あつい is the opposite of うすい, and ふとい is the opposite of ほそい.
As far as I know, 少し and ちょっと are synonyms which can be used interchangeably in most cases. However, ちょっと is more casual and less polite, so Duo uses 少し in more polite sentences at least.
Besides "a little", ちょっと is also used for negative things, something that cannot be said straight. それはちょっと… can mean "That's... (inappropriate/impossible)" and ちょっと、あぶないよ！can mean "Hey (think a little)!! That's dangerous!"