The circleOS is "multi-application", but if you wish to launch exclusively yours, you have to modify (slightly) the OS, and to recompile it. See in menu.c how an application is laiunched, and do the same after the initialization part.
For the second question, the best is again to analyze the source files of the CircleOS. The main problem would be that the systick frequency depends on the CPU frequency, But if you decide to modify the OS, you can fix the CPU frequency (you will have to choose between consumption and performance). I believe that for the medium value (default) of the CPU frequency, the Systick is called once per second. If you add a simple divisor in your application, you could easily find a multiple of the ms.
