"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.
Just ignore it. It wouldn't be a good example of either word anyway. You can wrap a ほそい string around your hand but not a ほそい pen or ほそい street. You can also wrap an うすい cloth around your hand but not an うすい book. It's about the degree of thinness/thickness, not which word it is.
Just remember that ほそい and ふとい are for long thin and long thick, and うすい and あつい are for flat thin and flat thick.
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!"
How would you say in Japanese "this shirt is a little too thin"? This was my translation of the given Japanese sentence as the literal translation "This shirt is a little thin" seems to imply, at least in my understanding, that it is "too thin", but my translation was rejected.
It can be used with adjectives and verbs in stem form; sometimes with nouns too.