RE: stuff on the bottom of the bucket -
I just take my spoon (or whatever I'm using to scoop out the feed) all the way to the bottom of the bucket and scoop up from there to get a good distribution of everything. Mine has fish meal and the mineral mix (fertrell) in the bucket. It all goes into the feed, but I do mix my own now. RedRidge separates out her fishmeal/powders, but I haven't ever done that. I just put it all in.
The Commercial ff
Is your regular feed pellets or crumbles? I'd be interested to see you try an experiment. You may have already done this so if you have, you can just say if they react differently but... Since you feed the scratch on the ground, I'd be interested to hear how they would react if you toss some on the ground. Will they eat it differently from the ground than in the feed pans?
Regarding "White Scum" on the top...FYI
-The White Scum" you describe is
Kahm Yeast.
Some people have mistakenly called this the "mother". That is INCORRECT.
You can usually smell that yeast/mold smell when you have it.
Here is a representative photo of kahm yeast growing on fermenting tomato juice from
http://www.wildfermentation.com/raw-tomato-preserves/kahm-yeast-on-tomato-juice/
You can see that it is kind of like a light "web" growing on top.
The Kahm yeast won't hurt the chickens (or people), but it WILL change the flavor of the ferment and the chemistry in your bucket if left, causing the
yeast/LABs ratio to get out of balance.
When lacto-fermenting food for people, you regularly skim any kahm off the top of your fermenting foods as it really does change the flavor...most people wouldn't want to eat the products when they have this...your chickens probably won't care for it either. It is usually quite preventable if the fermenting food is well under the liquid in the vessel but if there are any "floaties" it does enhance proliferation of the stuff.
When I'm planning on fermenting long-term, and because I do LACTO-FERMENTATION, I don't want the yeasts to get the upper-hand in the bucket. I skim off any kahm and take action to change the balance back to the LABs being in dominance.