Your cuttings are doing very well. I hope your cups have drainage. when you start seeing root development transplant to a larger pot - 1 gallon. You will probably need to transplant again before winter. Don't fertilize until next spring. that would push foliage growth and sacrifice root development. Wait until they are 3' tall and either in a 5 gallon bucket or in the ground before starting to shape the tree. Don't be too concerned about staking them. Putting a stake in the container will damage the delicate root system. Turn them every few days so all sides get the same amount of light. They leaders are tender. They will straighten as they develop.
I love crepe myrtles. Which ones do you have? My favorite is Dynamite- the only TRUE red crepe myrtle - fire engine red. One of the smaller trees - 12'-15' tall X 10'-12' wide. Another favorite is Natchez - Large (25'-30' tall) with a snow white bloom.
When they are ready to be transplanted into the ground make sure to allow for mature size. Dig your hole no deeper then the root ball but 3 or 4 times wider - a $5 hole for a .50 plant. Don't amend the soil and don't fertilize or prune until it has been in the ground for a year. It needs that time in the ground to develop an extensive root system.
I am going to edit and post a link on proper pruning techniques for crepe myrtles. It makes me crazy to see "crepe murder" committed on these beautiful trees.
https://www.lsuagcenter.com/news_archive ... efully.htm
BTW - if you have a deep color don't plant it where the blooms will fall on the concrete or a vehicle - it will stain.
Good luck