As far as rigging and fitting the clothing, it doesn’t matter whether you weld into one or leave as separate - Studio reads it all as one. On the one hand, keeping them separate makes UV mapping simpler - you must UV map before exporting to rig. On the other hand, if you ever need to apply the smoothing modifier to get a better fit, the pieces will tend to separate from the clothing. If it were up to me, I’d weld all together - not just the groups, but physically attach using weld vertices.
If you have a belt, rather make that a separate clothing item - especially if it has a buckle that you won’t want to have deform with the bending of the body. You can do this by hiding it when you export the clothing and it won’t be part of the .obj.
For making creases, after everything is done and saved as an asset, you can send it over the bridge to Hex in the posed position and model the creases, send it back and save as a morph. The creases will only show when you invoke the morph. Remember to have the resolution in the base mode.
You can also use other software, such as Zbrush to sculpt the creases - a very basic, but good (and free) sculpting application is Sculptris. To use this, export the clothing in the posed position as an .obj, do the sculpting and import it via Morph loader.
I would do the texturing after rigging and before saving.