The recipe that you may use in commercial baked goods will be regulated by your local health codes. If you are a commercial bakery, then your foodservice supplier can provide you with mousse base mixes and fruit concentrates.
In general, goods made with milk or cream must be refrigerated. This does not prevent them from being used on a cake covered with fondant, but it does mean that you have to plan carefully to ensure that the cake remains cold while you are covering and decorating.
Making a mousse stiff is easy--use gelatin dissolved in liquid. Warm over hot water until it is clear. Whip the clear liquid gelatin with other ingredients.