Last year we (University of Maine) completed a new planetarium and observatory. Our planetarium design is similar to your plans (we hold about 70 people); however, we have astronomy labs that meet weekly. With the addition of the new observatory we wanted astronomy lab students to have direct access to the observatory. We added a multipurpose room to the planetarium that can be used as a classroom (it has dividers to separate it from the circular entry room) while allowing planetarium shows to run at the same time.
You can check out the Emera Astronomy Center at http://astro.umaine.edu <http://astro.umaine.edu/> . There are links to a tour and a description of the equipment in both the observatory and planetarium.