cardarine can be stacked with anything . its not really 'synergistic' sort of speak with AAS because the mechanisms of action and receptors are completely different , but 'on paper' cardarine should help with your fat burning and muscle retention goals.
it binds to the PPAR receptor, and recruits an enzyme, which then increases the expression of genes involved in the expenditure of energy (which would increase fatty acid metabolism ). also , fat loss from cardarine is a result of increased glucose uptake in muscle. Essentially (again on 'paper') you're making better use of the nutrients you consume every day and decreasing the amount of carbohydrates or fat stored as adipose tissue. thus not only would it help you have more energy , burn fat, it also acts as an anti-catabolic.
i say "on Paper" , because I have not used Cardrine (gw50) and so my thoughts are only based on what I've read. I will be testing these things on myself soon though