Relevant question of yours. Well, the thinking is to avoid disk access when FSX is running. Hence the need to keep the OS 'there' and FSX 'somewhere else'. Physical borders. The OS always does something on the disk.

So it doesn't matter if you put other things than the OS on the FSX disk, as long as the OS can do its stuff and, when FSX is running, no other programs access the special FSX disk. The only downside with placing other programs on that disk will be the loss of space (not very relevant with drives >1tb) and the increased fragmentation (slightly relevant, but manageable). The latter being a pro point for SSDs i've missed so far, they don't need to defrag but it's not like mechanical drives (especially the ones offering much free space) demand a defrag run every week or so.