The specific technique you are practicing is meant to strictly alternate but that does not mean that this is the only technique worth learning. Picado is strict alternate picking. Anchoring is used in this technique too, but imo this is not necessary. By anchoring I mean supporting the right hand by placing the thumb or free fingers on an adjacent string.
Though not strictly Flamenco, Parkening and Segovia have exercises for developing alternate fingering that use every combination of fingers. The Segovia scales in all keys are meant to be played i-m, m-i, a-i, i-a, m-a, a-m, i-m-a-m, p-i, i-p, p-m, m-p, p-a, and a-p, each pattern used for ascending and descending, each done tirando and apoyando. Parkening has several chromatic exercises in his beginner book that follow similar approach to developing right hand technique.
If you are resting the fingers on the adjacent string it is natural to want to use it, like economy picking, but as Parkening points out in his book you are not supposed to "rest", you are supposed to bounce you finger off the next string slightly (my interpretation of his description) thus preparing it to move again. This is sort of like Buddy Rich drum stick technique, or Jun Fan boxing, you move your body back to the original position as fast as possible after the attack.