What are the benefits and drawbacks of low fat museli with one spoonful of low fat yoghurt? Will it lead to excessive weight gain?

Not the key. Weight is ultimately about calories in and calories out, but it's a fact of life that nobody is really counting. The key to staying trim is a fitness-focused lifestyle, including exercising and choosing friends who exercise. Avoid very greasy, very salty, and very sugary foods as best you can, and learn to limit portions. Exercising helps your brain regulate how much you eat. Best wishes.
Limit amount. Depends on how much of the museli you eat. If you eat a cereal bowl full with only the tiny amount of yogurt you mention, that will certainly not cause you to gain weight. Low fat yogurt, being a healthful probiotic, is a food you would do well to eat more of. If the yogurt is loaded with sugar, that will not be as healthy. Museli is healthy, providing good nutrients if not eaten to excess.