Do these questions make any sense and could someone not just answer them, but also, recommend videos/books for a new comer to learn more about these kinds of things? (I had come across someone asking these things after watching the Geoff Lawton food forest lecture, so I'm quite ignorant and looking to be pointed in the right direction).
Will my bacteria migrate from vetch and cowpea to my acacia? I wonder if I can inoculate my acacias under my walnuts by planting vetch or clover with inoculant? I suspect they will migrate to the acacia root over time or after the small legumes die? Or are they different species?
Bacteria will normally be in the surrounding soil as well as on the roots. Bacterial interactions are different than mycorrhizal interactions the mycorrhizal organisims attach to the root where as the bacterial organisms move close but normally do not attach themselves to the roots. There are many different species of both, mycorrhizae tend to be less plant specific. To identify either you would need a microscope.
If you are not certain the easy way to inoculate is to simply put some of the root structure and surrounding dirt from them into the soil so that the harvested roots are in contact with the new plants roots.
The Cavet here is this; mycorrhizae are present in all undisturbed living soil, the idea of inoculation is perpetuated by folks in the business of selling specific, not necessarily correct mycorrhizae.
Here's some interesting facts about the craze in mycorrhizae currently going on:
If your soil is supporting plant growth now, it is possible that all you need to do is amend the soil so it is optimized for the plants you desire to grow there.
If the soil does not support life now, then there are no mycorrhizae present, if it does then they are already there.
Inoculation of soil does not work as well as (or in most cases at all) inoculation of seeds.
Healthy soil usually already contains all the organisms needed for plant growth. If you till it, you kill it.
Mushroomcompost is one of the best amendments you can add to the soil, it is rife with all the desirable organisms you want in your soil.
Fertilizer is a guaranteed killer of both Bacteria and Mycorrhizae.