Re: current regulating diodes:

Surface area of a cylinder: A = 2 π r h + 2 π r 2
Stock = 3" @ 12 AWG 
12 AWG = 0.08080" dia.
Solution: A≈0.77 sq in.
Conclusion: .75 to 1mAh is acceptable 

ELECTRODE RATIO 
On the topic of anode /cathode ratio's, a visual differentiation is most often all that's needed. One common method of doing this is to gang several wires on a silicon stopper and use a smaller dia. cathode. 

CATHODE MATERIAL
As for using other materials for the cathode, I used to use food grade stainless wire until I discovered that it affected the taste of my solution. And so I stopped doing this in favor of silver electrodes all around. However, I'd add that my own method of making CS involves processing at 105c, and so this may have contributed to the results. - your mileage may vary. 

OXIDE FORMATION
Correct. In my opinion, any cross contaminants such as the formation of oxides will come at the cost of the particle size/quality. That said, the formation of oxidation on the cathode is consequential to the electrolytic process. However, there are ways in which you can eliminate this either by, heating your solution to the point of reducing the ions into elemental silver and/or with the use of additives, as well as a few other not so popular methods.

  PS. I'd also recommend(if you haven't) the use and admission of a multimeter to monitor electrical parameters during processing. ie, one multimeter is helpful whereas two is ideal as this will allow the monitoring of voltage and current as things move along.