There shouldn't be an issue using SunEdit2k (or TibEd) if you click on "Single player" in the launcher or use the "Tiberian Sun Game" shortcut it creates if you installed the full version of TS. This will start TS normally just like "back in the day".
Otherwise you have to simply rename "rules.ini" to "spawn.xdp" then it should work in the CnCNet client as well, if you want to play with other players using your mod all the other players must have downloaded your spawn.xdp file and placed it in their TS folders.
I always recommend editing rules.ini with a text editor though. Editors like SunEdit 2k or TibEd are kinda notorious for adding a whole lot of stuff to your ini files that isn't necessary and could cause problems while modding.