Can i use it as a cake filling? Do i have to refridgerate the cake then? Or leaving it in room temperature is fine. Thank you in advance

I would think if it has cream in it that you would have to refrigerate. Isn't chocolate cremeux like chocolate cream? 

Cremeux is very soft and would need to be dammed with a strong buttercream. And yes, it should be refrigerated until approx 1 hour before serving.