Whatever you were using for your CD (or "box") version of FSX that you like will probably work in Steam also. The big difference in the two as far as add-on's go is where Steam hides it's version of FSX.
Steam tends to bury it's version of FSX deeper in it's directory structure than MS did. IE:MS FSX you install in, let us say D:\, so you get D:\FSX for it's main directory. Steam it's more like D:\Steam\steamapps\common\FSX.
As long as you can point autoinstallers (which is distrust deeply) for add-ons to the proper folder(s), then most add-ons work exactly the same. Click here for more info.