Whether hard or soft, all pastels in this line are compatible. For fine detail, NuPastel; for medium hardness, Rembrant, Grumbacker, Holbein; for very soft, buttery, Schminke, Sennelier. There are many others coming on the market almost daily. I don't recommend buying just one brand. Every brand has a different color range. Therefore, purchase a full set of a hardness/softness you generally like, then buy sets or individual sticks to test what other colors you need. I have sets of the 5 most accepted brands and none of the colors match exactly. I may love some blues from one brand for instance, but need to fill-in with another brand due to the color differences.
Unlike wet mediums, when you pick up a pastel, it must be the color you want (or combination thereof) so you need many colors to work with. If you are a beginner though, I would limit your colors to one brand, then add as the need arises and affordability is optimum.
Also take into account your surface; whether smoother or more heavily toothed, as well as the tint, and how you stroke. Do you blend alot or are you a fast, heavy stroker? Do you build up a surface with pastel or work alla prima?, etc.