Having done Little Shop about a year ago, I completely agree with Footer. The plant is the show. It's supposed to be from outer space, and therefore look like it. By simply using an actor/actors, the essential
plot device is weakened. The plant is supposed to start out as a baby, and then grows throughout the show.
In its traditional first and second size, it needs to be able to move, but is traditionally nowhere near big enough for an actor to physically be the plant. If you are sticking with the general sizes given by the script, the plant grows two more times, and obviously consumers people whole in the climax. The size of the final plant would probably be much larger than a single actor and therefore some sort of costume with multiple actors would need to be used. At this
point, all you would really have is a very awkward puppet.
The other thing would be maintaining some sort of recognizability between all of the sizes of the plant. That's easy with the smaller sizes, but the transition from the smaller sizes to the larger ones in terms of design of the plant/costume might be difficult.
Honestly, I think this is one of the things that you just need to spend the money on to
build/rent. Of course, if you are planning on doing the show in a very stylized manner, your idea may be a great one. My response was from the
point of view of a traditional production.
(Then again, this is a response of a high school student, and therefore, take it with a grain of salt.)