Hola Femi,
1) The slab tool, by definition, only creates flat, horizontal surfaces. So, to put a "soffit" underneath your gable roof, assuming this soffit slopes upwards towards the ridge, you'll want to use the Roof tool, used for any sloping surfaces (not only "roofs").
For example, perhaps you can copy/paste the roof you already have constructed, then drop the elevation of your copy to the soffit height, change the thickness and materials for this soffit, and cut a huge hole out of this copy representing the footprint of the house, so that only the exterior overhang of the soffit remains. (but it will be sloping on the non-gable edge as well -- use the slab tool for this area of the soffit if you want it flat)
2) I think you're referring to the annoying feature where the dimension numbers "jump" outside of the hash lines that define the dimension area. This happens when the text is too large for the space. The solution is to hover your mouse around the bottom left corner of the dimension text -- this is where the "hook point" is for the text. Once you highlight the dimension text, you can manually drag it where you want.
Hope this helps. Good luck