Specialize or not?

It all depends on your markets and your storage ability. Ferrous scrap is far less valuable, requires far more space, but is also far less volatile versus nonferrous. So while nonferrous may be riskier (copper can gain or lose 10-20 cents in a week, especially in the geopolitical environment we find ourselves in today), if you're smart and thoughtful, and lucky, the reward can be significant. I think the best thing you can do is to cultivate relationships with your local scrap yards, figure out market movements and indications from them whenever you can, and just buy whatever you can at the best price you can. Perhaps most importantly, don't be afraid to hold onto material for better pricing in a week or two when market winds shift. If you can find a good buyer at a good scrap yard, you should be able to get all the information and help you need to make the most money you can.

Speaking from a personal perspective, although I work for a large multinational scrap company, I always try and make sure the scrappers I buy from understand the markets as much as possible so they can make as much money as possible...I'll even tell them sometimes to hold onto that copper for a week if I think prices are about to go up. It doesn't benefit me to screw anyone over; you should look for buyers/companies who want to help you, not to screw you over for a couple extra cents of margin.