I keep hearing 50/50 mixture of rubber cement and thinner.  Where did
this advice come from?  I find this 50/50 mixture to be far too thick
to properly rubberize pouches.  (The pouches would be so heavy they
would hardly move!)

I use a mixture of 80% thinner (naphtha)  and 20% rubber cement.
Two very thin coats are usually sufficient.  There should be no
visible trace of cement.  All that is needed is to seal the pores
of the leather.

Dusting with talcum powder is asking for trouble.  Talcum contains
starch, and possibly other attractive things which might encourage
insects or encourage mildew to form upon the leather.

Use pure talc to dust the pouches after the rubberized treatment.

Bruce Clark