As the Hierarchy, you can study only six research branches. Study until you have brute pod, lost one pod and phase tank pod. Place them on your appropriate walker, then undo all of your research so you have none. Put those six in ones you want. Since you are building these pods they don't go away, so you can make brutes galore and don't have to have the branch required for it. Just remember when the pod gets killed or replaced you cant build another one.