Is there a potting mix shortage in your part of the world?
Seriously though, filling your pots completely will provide vastly more room for your plants roots, which translates into faster and more luscious growth.
Being so deep in the pots also stops your plants from receiving any reflected "side" light, again limiting growth potential.
You will also have to invest in some proper "grow" and "bloom" nutrients, there is no nutrition in the perlite/coir mix and very little in the root starter type of substances. Also, I wouldn't bother with the 125 watt CFL, your 400 MH is more than adequate and there is little to be gained by running the CFL, apart from a bigger electricity bill. 18/6 is plenty and optimum for cannabis growth, so there is also a little extra you can save there too.
If they are already flowering, there is little that can be done anymore regarding their size, and you are correct in predicting there will only be one stalk.
Next time, fill the pots right up and feed them with proper cannabis nutrients and I am sure you will enjoy a much greater harvest.
If you are after some recommendations for nutrients, all of the following are one bottle complete nutrients, meaning no mixing or extras to buy.
Organic "Grow" and Organic "Bloom" made by BAC;
"Bio Vega" and Bio Flores" made by Bio Canna and,
"Flora Nova Grow" and "Flora Nova Bloom" made by GHE.
All of these work superbly and are very simple to use too. They are quite strong, meaning one bottle will last many grows or multiple plants, as you don't need to use very much, making them quite economical too. Being so strong, half recommended strength with any of them, is plenty powerful enough.
Hope this helps,.....
Organoman.