I thought you summoned the whole package? If the summon category restriction only applies to the initial state, than the infantry spell could be used to summon horse, chariot, or even monster-riding characters, since in all cases then the mount would be a valid upgrade option for an initially infantry based character. And that seems even more incorrect than allowing the other summons to summon characters in the first place.
the 'what you can summon' restriction is dependent on what casting value you go with. If you go for the max sig cast, you get monstrous infantry instead of regular infantry, not in addition to it. so the fact that a different casting value of harbinger summons characters has no bearing on whether the casting value for monsters allows you to do so.