Suggestions for writing short papers?
First, follow Tara Bs advice of reading such journals (this will help in choosing the appropriate one as well). 
Second, try to describe your idea on a level of complexity, you would explain it to your grandparents in max 10 sentences (assuming they are no experts in your field). This gives you a nice outline of the most important concepts and context of the idea. 
Now dig into the topics which are really relevant, and explain them in more detail. Keep the state of the art as short as possible (but demonstrate that you know the relevant literature). 

Since you have no simulation results yet, your simulation system is the result of your paper. Therefore "methods" are the way you built your system and design considerations. In the results, you can show some data from pre-tests if available.

You should keep your major paper in mind, especially make sure, the full paper will not be regarded as self-plagiarism of the short paper. But if everything goes well, you can cite your short paper in the methods section of the long one.

In my field (computer science / informatics), one would often choose a conference paper instead of a short paper, but this really depends on your scientific discipline and strategy of your institute.