It is recommended that you download evaluation kits of simple and full kernels and try structuring your application for each. A little homework up front may save a great deal of unnecessary development work later.
There are many simple RTOS kernels on the market. The presumed advantage of these kernels is they are easier to learn than full kernels. The downside is missing capabilities needed by an application end up in the green application code rather than in the proven commercial kernel code. The less you have to design, code, and debug, the more likely you are to meet project goals. The cost of reinventing the wheel is likely to exceed the difference in cost between a full kernel and a simple kernel—even if the simple kernel is free. It may not be obvious what features your application needs.