Hi Hunter and welcome. Locusts are very peculiar trees. They have strange habits that make them a bit difficult to work with. The main item is the tendency to abort a branch that you are working on
. Trimming must be done with care and over time. New growth can be tended to without much care, but when you start working on older established branches you can not whack off big chunks or the rest of the branch will die back. You have to sneak up on them and take little bits at a time and leave a sap sucker to keep the juices flowing. After the branch makes some new shoots the sap sucker can be removed. They are pretty hard to kill, but they will sure give you a run around.
Speaking of run around! The young trees are voracious rooters! I've had younguns push right out of their pots. Lesson learned, avoid small pots! Give lots of lateral space to spread out their roots in. It will take about three root replacements before the juvies settle in and calm down. Best to grow them in the ground or large box.
Due to the leaf structure you're best to be thinking rather large bonsai as even when reduced you're still looking at a rather large compound leaf. You can cut the fronds to shorten the leaves and shape the canopy. Try not to get to attached to any one look or design and be prepared to redesign on given notice. In other words, try to keep up and roll with the changes.
I am speaking of Black Locust which I have out the wazoo. There's an old four footer standing next to the house with kids and grandkids all over the yard. That and Enghlish Elm I got!
I have found one cultivar that is very well behaved. It is a Contorted Black Locust and she is the darling of the garden. It is called Lace Lady. The trunk twists and turns and the bark is thich and fissured. The leaf stems are not stiff so it makes a weeping tree. From away you see a delicate lady swaying in the breeze and when you get closer and see the stength and power of her twisted furrowed trunk you know she's not just another pretty face. It would top out at about eight to ten feet if left alone, but I reduced it for the first time, after three years growing. It was developing into a forked trunk and I didn't care for the look. Scared the crap out of me! It took so long to pop bud this spring I was beginning to think I'd jumped to soon. One of the freaky things about Locusts is the they don't hold bud like most deciduous trees that you can see all winter and watch to see when to move on them. Locusts look completely dead all winter and new buds pop along the stems as the sap rises. Waiting with bated breathe on that one this year, but she finally sprang to life and responded nicely with much new growth! Whew!
All said, the natives are a bit restless, but the cultivars are more user friendly. Best of luck taming that wild thing, Rick