Are the tile assets from crayta allowed or do we have to use our own assets, and does it affect our rating if we use our own

Not sure what you mean by your own assets, but you are free to use anything available within Crayta or that you can upload to Crayta, such as custom UI images. Pleasr let us know if this answered the question.

It slightly has, but does some of the scoring aspects add up if we add custom ui, or is it just an optional choice

Custom UI would definitely improve your score, and particularly for the UI award!

However it definitely is not mandatory