All my crape myrtles were about that size when I planted them. Pruning has been an ongoing thing. I let them develop at least 8-10 limbs before pruning off the bottom row of limbs (while they are dormant, usually around late December here). Mine all have several trunks as this is the look I like. They are all 2-3 years old now, approx. 5 to 10 feet tall and they are all shaping up very nicely.
I have some crepe myrtles (white) already in my front yard but recently added to them so there would be a row of these beautiful white flowering trees along the fence. I now have 6 total. When I purchased the 3 new ones they did have small branches up and down the trunks but I wanted to have them look clean except for the top. I did cut off these little growths and have potted them to grow new crepes. ( I love growing from cuttings) Anyway, one thing that I was sure to do was spray the trunks with this product to keep from having more growth where I removed the previous one. It's a black spray that keeps germs from invading your plant at the cut and also keeps it from growing again in the cut area. I dont think its a bad time right now to trim the growth to start shaping your tree but I would wait for major branch cutting till it's dormant just to be on the safe side.
Usually it's not recommended to use pruning sealers like that to cover pruning cuts, people used to recommend that approach but the general thought now is that they can do more harm than good to the tree. It's better to let the pruning cuts heal on their own.
Edited to add--if you prune the side branches off properly, they shouldn't put out new growth anyway even without the sealer.
Guess Im still old school cuz so far my crepes and other trees havent shown any neg affects. My purpose for not wanting any lower growth is to get a tree instead of a bush out of the crepe. The first 3 trees have done very well with this. Hopefully the 2nd 3 do the same
I think a lot of times you could use the sealer and it won't do anything bad, it's just that if it's going to have any effect at all, it's more likely to be bad than good. Most of the time it may not really do anything. But as I said, if you prune the lower branches off correctly they won't resprout and as long as you used clean pruning shears they won't grow bacteria even if you don't use the sealer, so I think you'd be fine skipping that step (and if your pruning shears weren't clean and you do use the sealer, you could get bacterial growth and now it's sealed inside the cut where it's easier for bacteria to grow than if they're exposed to the open air). Where you'll get the new branches that you have to constantly fight with is some crapes (maybe all?) will tend to send up sprouts from the base, with those there's really no good way to stop them, you just have to cut them off as they come up.