It is appropriate to use "plan" here, but it looks slightly odd to me as a native speaker and holder of a B.S. in Biology in this specific context. As noted in comments, phrases like "mammalian body plan" are not only common but are the accepted scientific terms across the field and especially in anatomy. "Design" is commonly used in this context in the field (in my experience and observation, but I have never focused on anatomy or physiology, where this usage of "plan" is standard).
Outside of anatomy and physiology, a plan generally implies intent from a planner making decisions in advance to support some specific outcome. Similarly, a design implies a designer but the implication is less strong than with plan. Scattering paint randomly across a canvas might produce a pleasing design, but would never produce a plan, despite the actual result being neither planned nor designed.
Evolution operates on existing structures or processes and favors certain changes over others in different environments, leading (in very, very general terms) to structures that are effectively designed to operate in a specific way (or ways) for a specific purpose (or purposes). When there is a single structure, like a digestive tube, that has (essentially) a single variant, it is "designed" in the sense that evolutionary pressures favored that one variant and not others. But this is an unguided process that does not require intelligence or intent.
In summary, a biological structure has a plan/design that describes its actual structure: what components it has, where they are, how they operate together, etc. You can describe a biological structure as being designed or having been designed, such as for a particular purpose, but trying to describe that same structure as being planned or having been planned for that same purpose would sound odd. If you are using "plan" in this field as a noun, you may be (and probably are) using it correctly. Using "plan" as a verb in this field is probably incorrect.