What species should I be using in zone 7b here in GA?

I purchased cover crop seeds last fall that contained multiple species of nitrogen fixers and deep tap rooted plants for breaking up compaction. I am two years into creating my forest garden, and I am trying to keep some areas available for annual vegetables. I now find myself chopping and dropping (mostly vetch) these plants around the base of my trees and blueberries. I had thought about allowing alternating rows to be filled with nitrogen fixers. The vetch seems to be very aggressive and is probably not my best choice. I will wait and see how it fares during the heat of the summer. I realize that most people are using comfrey, but I have heard that it is also rather aggressive.

I see you aren't getting a lot of answers here, so I'll jump in although I know nothing about your zone..

OK I'm in zone 4/5 ..Michigan..I find that the best things for me to chop and drop are things with large leaves..

I use a lot of comfrey, and also use the leaves when I harvest rhubarb.

I chop down the large leaves of my horseradish plants as well to use as mulch and I go out in the byways on my property and gather things like milkweed as they have large leaves as well.

I have read that you can also cut small leafy branches off things like willow and alder etc..to use as mulch..i haven't done that yet..but would be careful not to BURY anything that might root in the garden esp the willow branches..just lay them on top.

I have no hay or straw available easily around here as there is a huge hay shortage from Michigan's drought, so I have to make do

I am looking to use comfrey but I plant to grow in one area and mow with bagger then move mulch to another area.
I would also like to try this with burdock. I would mow in second year just before the flowers form.
It seems to me that you do not have to chop and drop exactly where what you chopped grows.
You can also chop and haul the mulch a short distance to where it is needed.

If you don't mind annuals, I like peas & beans -- stacking functions -- fixing nitrogen, generating biomass (chop & drop), edible (pick what you want), and reseeding (about the easiest seed you can save). I like to scatter some oats so they have a little support. Oats don't stack quite so many functions though.

I'll second comfrey and burdock, just chop before they make seed. Horseradish always comes up in these contexts and it's on my list of things to try.

A short-rotation annual is buckwheat. Attracts beneficials, suppresses weeds, makes decent biomass. Reseeds if you let it go that far, but doesn't make itself a nuisance if you chop it before seeds form.

Lastly, look around you: what do you see growing vigorously at forest edges? Are there plants that you could dig up (or gather seeds from) and add to your garden that are suitable for chopping?

Amorpha is a nitrogen fixing shrub that can be used for chop and drop around annuals...I've seen in done around Athens, GA and may work for you.

green growing limbs under 3" dia, sometimes called ramiel wood, can be used as mulch (chipped or unchipped)
smaller the twigs the better prune those invasives!
this wood contains nitrogen and other nutrients that leach into the soil
i recall an old Permaculture Activist magazine where studies were done showing improved growth using small (<pencil size i think) twigs placed between plants

Duane, that is what I hope to do this year, as I have also read how beneficial they are and I have a lot of alder, willow and aspen available..as well as branches of my nitrogen fixers like baptisia and autumn/russian olives.

I lived in GA for 20+ years. Your vetch will die out when the hot summer comes in, usually scattering enough seeds to come back next fall-winter-spring. It's probably good mulch, if you can stand it's vininess. A rainy summer will make just about any mulch disappear though. Comfrey you will have trouble getting through the summer, unless it gets shade and moisture. Buckwheat is a pretty quick-to-finish annual and not very vigorous, unless you're up in the mountains. Mostly I mulched with pinestraw and oak leaves from surrounding woods, and I also peeled the bark off of all my firewood for a long-term mulch. And I used the common mimosa (Albizzia) as a coppice, nitrogen-fixing interplant.