Even the 500 mg/week dosage level, as a total of all the steroids used in the stack, is sufficient for substantial improvement in fat loss compared to the natural state. There’s some further improvement as the dosage increases to about the 1000 mg/week level. If there’s further improvement to be had past that, I don’t find it noticeable or certain.
In a few cases there have been remarkable results with quite low dosages, such as 250 mg/week testosterone or even 9 mg/day Dianabol Methandienone (as odd as that number is, the specific case really was that amount.) However, that’s unusual, and appears to be correlated with the individuals having somewhat low natural testosterone. Generally, 500 mg/week is a reasonable minimum for a fat-loss steroid cycle.
Whether to lose fat first and then gain muscle, or do it the other way around, will depend on the case. A simple rule of thumb though is to accomplish the personally-easier task first.
For example, if you know you can drop 10 lb of fat relatively easily but adding 10 lb of muscle will be a challenge, then by all means lose the fat in the first few weeks of the cycle, preferably with quite high volume training. Your body will then be in a highly responsive state for muscle gain in the following weeks, due both to the previous high volume training and due to a homeostatic tendency to return to previous weight, in this case with muscle gain. More importantly than any reason why, this simply has been found to work very well.
Or if on the other hand if you find it hard to get much leaner than your present condition but you know you can add the planned amount of muscle in a matter of weeks, then add the muscle first. This will aid in the following fat loss, both because the added muscle increases metabolic rate, and again because of a homeostatic tendency to return to accustomed weight, in this case with fat loss. And again, regardless of reason, this too works very well in practice.
Gaining muscle while losing fat simultaneously can be done, but generally isn’t the optimal approach. I would pretty much look at that as being a possible unintended outcome in some training scenarios, where very intensive programs with intent of maximum strength gains might result in some fat loss despite best efforts at sufficient eating. But as a deliberate plan, most times I wouldn’t aim to accomplish both fat loss and muscle gain at the same time in a steroid cycle.