Hey Smeg..questions always welcome...
You can increase the amount of pebbles in the ratio which helps give a nice loose mix.
I mix coco/perlite 1/1, and would keep same for a coco/pebble mix. I always soak pebbles overnight @ pH 5.5 and rinse with same before use. The reservoir may get overly warm in the wardrobe. If leaving it there I would insulate and cover with reflective material. Increased temps can cause bacterial issues in reservoir and its location may add too much humidity and require situating res outside robe...
I'd advise venting filtered air away from tent to keep RH in check. Once in flower venting through door would cause a stink. I vent mine through insulated ducting to eaves of house..
What output is your airpump...??
Hope this helps...good luck.