Hey!
I can see why you are temnpted, but to be honest I'm not sure it would work too well? I don't mean to be all negative, but there are a few things you should bear in mind if you decide to go this route:
Slim fast is designed to supplement 2 meals per day with the third meal being carefully balanced/healthy with less than 600kcals (I think?!).
Slim fast doesn't contain all the nutrients per serving that we benefit from when doing CD - this could have implciations for your health.
Slim fast is also much higher in carb content than CD - compare a shake with a slim fast serving and you'll see the difference is massive. Not to mention that the calorie intake is much higher if you SS on slim fast you are having 200ish (i think) calories per serving compare to 138 ish with CD.
Slim fast (due to the carb content) doesn't have the security blanket of ketosis - you are far more likely to stray off plan due to hunger etc
I think you would need to reduce the slim fast powder portion drastically to get anywhere close to the CD calorie & carb content. Also worth bearing in mind that there is no structured plan towards maintenance with slim fast. Wouldn't be much fun to lose the weight and then pile it back on when you finished.
Best of luck if this is what you decide to do, but if you can afford it CD is a much safer approach. Alternatively, why not look at maybe the JUDDDD?
xx