Seems reasonable enough. Fleet commonality is based on formulas/calculations and having a separate box showing the changes in fleet commonality costs when you're acquiring a plane would help out greatly. It's just like buying a car--you can research and calculate rough maintenance costs before you buy it.
It would be nice to see the fleet commonality costs when your going through the process of acquiring a new/used plane, but a much simpler option would probably be a link on the fleet commonality page with a "what if" scenario simulator where you can select a plane+engine to see estimated costs of adding said plane (on a per plane and aggregate basis). This would really make people question adding that 3rd/4th/5th fleet type.