It's going to be a 8 inch and a 12 inch cake.
What do I use at the bottom, a 14 inch seperator plate or a cake drum?
I am afraid of the cake being heavy for the plate.

Hi,
I would use at least a 14" cake drum or make your own by gluing together a few 14" cake boards and then cover with foil.

Separator plates go inbetween tiers. Cake drums go under the bottom tier to support the weight of the entire cake.

From time to time I have used a separator plate as a base.
That size cake will not be too heavy for it.
Overall, a cake drum will probably look better