give it a go.
the idea of using size 8 needles and worsted yarn when ou first learn is purely so you can get a hang of the technique. using yarn that is to thick or to thin (likewise with needles) can be challenging if you are trying for the first times (boy do i know it!).
however if you have played around a bit and tried the basic kniitting and purling etc and are happy(ish) with how to do them then why not go for some funk yarn and different needles.
There is certainly no set time scale which says when and how you can do things. you just go and do what you are happy and comfortable with.
I am Tarriknits on Ravelry
This is my not necassarily knitting related blog