Yes you would be way better off planting some clover or such. Alfalfa isn't good for grazing and won't hold up like grass will to traffic from animals. Plus the animals will most likely never give it a chance to get going.
As far as seed goes usually you buy certified seed. Certified seed has to be inspected and it is weed free. The way it is harvested is the alfalfa is left to go to seed. The seed has to be activated by using cutter bees. Once this is done then the seed is harvested with a special machine. The seed is certified once again. As a kid I spent many a summer day walking through the fields cutting weeds out of the alfalfa seed.
If you do it right and get the soil right and use good seed you can get 7-10 years out of a field of alfalfa.
With the oats you have two choices. You can either harvest them for grain or you can make oat hay out of the first crop. You just have to cut it while it is still green.