Has anyone purchased and used these molds? (see link) I'm thinking of ordering some, but I want to make sure they really are flexible enough to get fondant out of without messing up the design.
You need to make sure that the molds are made of FOOD GRADE silicone or plastic.
It doesn't matter whether they are flexible, if they are releasing poisonous chemicals into your fondant.
FYI you can always freeze the fondant to get it out in one piece...
I looked at the page of one of those molds and it clearly stated it could be used for fondant and is food safe. If the description of the mold doesn't indicate if it is food safe, you need to ask the seller. If you have a food safe mold and use it, and then decide to use it for non-food (such as polymer clay), then consider the mold non-food-safe.
Have have ordered some like this from etsy (not the specific seller you have listed). The ones I got were food grade, and it is easy enough to get the fondant out
Looks like they're made out of Amazing Mold putty, which is food safe.
I use it to make my own molds for fondant and gum paste and it works very well. Plenty flexible.