What is wrong with Does dim rhaid i chi ffonio yfory?
That would mean "You don't have to" (= you may if you want but there is no compulsion) rather than "You must not" (= you may not do so).
I'm doing this lesson on my phone, so I apologize in advance (to kook at the notes) . May I not use dim for not? Must it be beidio?
Yes, when it is 'not' in the sense of 'not doing something' - refraining from doing something.
"Peidio" actually means "to stop or cease." As a result, the "rhaid i ti beidio" syntax can be semi-literally translated as "You must stop" or "You must cease and desist." In other words, don't do it.
I missed out the 'a' and it was still marked correct.
Yes, many people miss out the â in the colloquial language. It is 'Good Welsh' to use it, though.