You can look here on BYC in the Buy-Sell-Trade/Animals in need of free re-homing section for a free roo; also local newspapers, penny saver papers, etc.
A rooster of the same breed is only important if you want purebred chicks to later sell. If not, barnyard mixes work well.
For the safety and comfort of your hens I would look for a rooster of the same size or even a bit smaller than your hens; but not so small that he doesn't have a chance of a successful mating.
Look to the breeds that are known for their gentle temperments. Do you research and you can also find some info. here on breed temperments:
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html
After a quarantine of no less than 30 days, I just stick the rooster in with the hens. There's usually some fireworks to start with, especially if there's a very dominant head hen that's not keen on giving up her throne, but unless anyone is getting hurt, I let them sort it out.
To answer your other question, one rooster for 12 hens is plenty. You should get pretty decent fertility at a rate of 12/1.
