I work predominately with buttercream but I am going to make a cake in a few weeks covered in fondant.

I keep reading that you need to use a firm cake such as pund cake so it will hold up under the weight of the fondant.

I usually just use box mixes (Duncan Hines). Are these ok to use with fondant? If not, what should I make/use?

I have almost always used boxed mixes with my cakes. I add an extra egg and a jello instant pudding mix to the batter and that makes the cake firmer.

Straight box mix would be fine. There are many people who use straight box mix cakes under fondant.