I’m making cupcakes with Swiss meringue buttercream frosting and fondant toppers. How is it best to store these? I’m worried if I leave them out of the fridge, the buttercream will collapse under the weight of the fondant. But in the fridge, the cakes will dry out and fondant could melt? Any advice please? Thanks.

The answer to your question depends on the amount of time you are talking about and who you are serving them to.

When I make SMBC cakes for my family I fill/ice them the night before I need them and if it's cool I leave them in my spare room with the door shut to avoid any heat going in there if our heater is on (and to avoid any little people touching them otherwise).

If it's reasonably warm I put them in the fridge overnight in a box and pull out first thing in the morning to come to room temp. 

Officially I think the time SMBC should be at room temperature before you risk bacteria etc getting to an unacceptable level is 4 hours so I would never leave it out for longer than that if I was selling them to customers.

Sturdiness wise SMBC is very strong unless the weather is warm then it will get soft but still hold it's shape so the issue I see here is actually to do with the time rather than the fondant damaging the SMBC.

What sort of fondant are you putting on them? A disc/topper/ covering completely etc?