I will also attest to the folks at Green Cover Seed. Deeply knowledgeable in no-till, cover-crop methodologies and can design mixes based on what your desired outcomes are and what your current conditions are. I would encourage you to give them a call.
Green Cover Seed
Here's their YouTube channel if you want to digest the extensive library about different kinds of cover crops, their options for use, etc. If you hear the term BMR, that's a reference to a plant that has a brown mid-rib structure to its leaf. Keep in mind, these videos are targeted to farmers who can speak slightly different language. I learn a ton when I watch their videos ... only a fraction of which I can apply to my specific setting and goals.
GCS YouTube channel
Companion to alfalfa: perennial ryegrass, meadow brome, tall fescue, oats
Companion to buckwheat: cowpeas, grain sorghum, brussels sprouts, kale, broccoli, peppers, squash; buckwheat can be used as a nurse (companion) crop with summer seedings of alfalfa or alfalfagrass mixtures much the way oats are used with spring seedings
Companion to oats: nearly any legume, orchard grass
Hope that was helpful.