Over the years, we have installed FileMaker Server in many different version (including 11) on our servers and our clients' servers. There is nothing inheritly wrong with having multiple services (FileMaker, web, file sharing, print serving) on the same computer. It really all comes down to the hardware you have (memory being most important) and the workload you put on it.
Not knowing how much of a workload you currently place on your server I can't answer this question easily for you. My advice would be just to try it and if you notice degradation of services then dedicate hardware to FileMaker.
Austin Michael Internet Solutions
FileMaker Hosting Services