It is my understanding that consultants can be used wherever and however specified, inasmuch as they are bought as support, and that the buying criteria must be met for each squad.
Therefore, if a Capitol army wishes to have a squad of Brotherhood grunts, then they BH grunts are purchased as support. If an elite unit were to be purchased, then there would need to be two support slots available - one for the grunt to qualify the elite, and one for the elite.
I am not stating this as official, as I, myself, am a little unsure of this.