If you are not using perishable fillings or icings, your cake is fine at room temperature.
Sorry I don't understand what perishable means?
Perishable means it will spoil at room temperature
Like milk and fruit?
will butter spoil?
Fresh fruit will spoil, but if you use fruit jam in your cake it will be ok. Milk and butter, mixed with sugar to make buttercream, is usually ok at room temperature (it depends on your recipe and how much sugar you use...the more sugar, the safer it will be). Fresh cream or custard type fillings should be refrigerated.
Would you be able to recommend a tasty Buttercream that won't spoil?
Will fondant be okay at room temperature on the cake or should it be covered?
With all due respect, you should find out what your local (state/province/county) regulations are for either home baking for sale, or commercial restaurant food handling. Even if you do not advertise or seek formal certification, it would be a good idea to conform to your local regulations as much as possible.
Fruitcake is stable indefinitely at room temperature, and the traditional coverings of almond paste, sugarpaste and fully dry royal icing are also stable at room temperature.
There are a few general rules for other kinds of cakes:
Do not store raw food higher up in the fridge than the baked cake or its components. In some places a separate fridge is required for raw and cooked foods sold to the public.
If there is milk or cream or egg yolk in the filling/icing then you should keep it in the fridge at all times.
If the sole filling/icing is fully cooked (meringue, jam) then it is safe at room temperature for a "normal" amount of time required to decorate. In general, a cake would be delivered within 48 hours of assembly from baked layers. Hold it longer, and it loses that "fresh" taste.
Baked not-iced cake layers are good for a month in the freezer if properly wrapped.
If you must hold the fully iced cake longer than 48 hours, then it is best to freeze the fully iced cake. YES even if there is fondant on the cake. The instructions for freezing and thawing a fondant-covered cake have been posted on Cake Central on a regular basis.
Yes, I plan to do that.
I am also planning on buying a separate fridge/ freezer for the cakes.
.......... recommend a tasty Buttercream that won't spoil? ..........Will fondant be okay at room temperature on the cake or should it be covered?..........
Most cakes and American Buttercream icing can be left at room temp for 3 days (depending on how warm it is in your home!) Longer than that is best fzn, but I have kept cake in my frig for up to 10 days and still was good. Fondant covered cakes do well left at room temp also (w/o covering them until they are cut - then I cover the cut edges w/plastic wrap.)
A great, easy to make and use ABC is here: http://cakecentral.com/a/2-icing