Like most answers to this sort of question, there are no rules, just guidelines and you need to find out what works for you because we are all different.
If you want a ready mixed muesli then you need to choose it wisely. Check out the labels look for the lowest carbohydrate versions. Take care with those that contain dried fruit, chances are that will give you a big rise in blood glucose.
I gave up my favourite muesli very early on when I found that the dried fruit in it was the main reason for my post breakfast highs. Seemed I could tolerate the oats provided I kept the portion size down. Now make my own - its a granola rather than muesli - roasted oats with lots of nuts and seeds. If I eat it with natural yoghourt, and a drop of milk and I finish off with a brew and low carb toast. That way I get a quick and simple breakfast that satisfies my hunger and keeps my post breakfast high under to 9 which is normally back to a boring 5 after a couple of hours. As I say, that works for me, may work for you.