Bechamel is basically a white roux with milk added, reduced to a thick and creamy consistency. Seasoning to taste. You can add minced onions, nutmeg, or whatever else you want to flavor it with.
Veloute is basically a white roux + white stock (beef, chicken, fish, vegetable, etc.) Seasoning to taste. You can add flavors like a mirepoix, sachet d'epices, etc.
When you add cream and mushrooms to a veloute, it then becomes a supreme sauce ... a derivative of the veloute sauce.
Some sauces are intended to have a nice rich texture. If you want to reduce the richness, simply reduce the fat. While better for your health, it will alter the characteristics of the sauce. Enjoy the sauce, but eat less; or not as frequently.
Heavy cream is 36 to 38% butterfat
Light cream is 20% fat (range 18-30%)
Half and half is 12% fat (range 10.5-18%)