Hi some good info here all ready.. i'll try and fill in some bits although I am still learning.
Once you have an overall plan for your trees I think a really important part of the equation is the understory plants, cover crops, mulch plants, nitogen fixers etc.
for cover crops maybe look at red and white clovers, lucerne etc
nitrogen fixers such as lupins, vetch, broad beans, fenugreek..
Fill in the gaps with smaller productive perennials.. blue berries, goji berries, wild strawberries, currents and gooseberries.
Queen annes lace (seeding and flowering around eastern Melbourne at the moment, try edges of South Eastern freeway), borage, comfrey, tansy, pyrethrum, bergamont, echinacias and aliums are all great additions especially for beneficial insects.
You can play around with lots of annuals but a great idea is to establish a perennial and self seeding understory so it can manage itself and as a result should keep weeds as bay.
I think packing in the layers and diversity with a mind to succession planting is very important if not the key to establishing a food forest. If you have spaces or bare soil then mother nature will fill it for you so why not put something there you want.
