My cake is going to have 2 gumpaste figures as a topper. They will be on top of a 5 inch round pan iced in buttercream. I am worried that the weight of the figures will smash the buttercream. Is there any way to avoid this?

you could push drinking straws down in underneath them to support the weight. I use them as dowels all the time..they work great! HTH

You could place your figures on thick cardboard cover the card board with pritty foil or icing. THen put a couple Plastic dowels in the cake where the topper is going. That way the cake won't be smashed the topper stays in place and your goo to go. Hope that helps.
LadyBug