Hello, generally you don't need a rubble trench foundation for interior walls as the exterior wall's foundation will move any moisture away from the building. When I built my cob house I did 3' deep exterior and all the interior walls had shallow 6" rubble trenches just to help support the mass and define the areas, but they did not actually have any pipe or tiles placed within to move water out.
I do recommend building some type of stem wall. Again, this can be much lower than your exterior as you wont' have to deal with water splashing back from dripping rooflines. You don't want cob directly on the ground as it can draw up moisture if you are not putting a waterproof membrane beneath it. Better be safe and build up at least 6"-12" off the ground before doing any cob work for interior walls. More like 18"-24" for the exterior walls. Hope this helps.