Hmmm, I have never used the CS before. Is that a risky thing to do?
Not if you take a backup of the esp/esm you intend to edit.
Not sure what you mean by not setting it as the active file, but if it is fairly straightforward then I will probably figure it out.
To edit a mod, you must select it, and then click on the "Set as Active File" button. If you only check it, the CS will open with that mod, but any changes you do will be saved into a new file instead of changing the one you open.
This may sound fine, but doesn't work unless the mod you want changed is an esm file. If it is an esp, you must either make it an esm first (using Wrye Bash), or directly editing the esp by making it active, so for Real Sleep Extended you must open it as the active file and directly edit the bedroll. If you just check it, edit the bedroll's weight and save it to a new esp, this new esp will just contain an unused bedroll that is not related to RSE's bedroll.
But since it is RSE's bedroll you want to change the weight for, I suggest you wait for the next update instead. I have already planned to have a setting in the ini for the weight of the bedrolls, so if you don't like weight 10, just set it to 5, 2, 0 or whatever in the ini. The new version should be out later this week (after an update of Enhanced Economy).
And for the record, the adjustable weight will work by RSE reading the value from the ini file and then calling SetWeight, each time you load a game.
Edit: Answered before seing your edits. If the clothes come in an esm and not an esp, then you can open the CS with the esm checked, but not active, make your changes and save as an esp. This esp will have the esm as a master, and change the values in the master esm.