There are conpanion plants and cover crops. Some do both and some dont.
The only thing id be concerned about using alfalfa is how deep it will push its root system. I use alfalfa in my beds as a cover crop through the winter because its a very hardy species, this is due to its room system being nice and full, and deep. Ive never used it as a companion plant as ive never been advised to.
Some good companion plants that will till the top of your pot with ease would be red clover and buckwheat, flax, vetch, fenugreek, etc..
Basil is also good