I want to alter the behavior of a plasmoid. Is it OK to do this?
It's very easy to create a new plasmoid based on the one you want to alter. This approach then means that the modified and the original plasmoid can be compared easily and you are not limited by the authors of the original plasmoid's vision. The new plasmoid should be created in the playground area of SVN.
Instead of making kickoff be about applications, why not let tasks take enter stage in the start-menu?
i'd recommend a separate plasmoid to experiment with these ideas, even if it shares much of the code/infrastructure with the two other kickoff based plasmoids. you'll have a lot more of a free hand this way and we can more easily compare/contrast approaches, while not forcing people down one path or the other.