Hello Alex! Welcome to the forum!
The RS armor is a solid choice, and whilst we cannot say that it is a 501st approved armor, your chances are very high depending on your height and weight!
How tall are you and what do you weigh? I'm 182cm tall and weigh around 94kg, and my RS armor fits me perfectly (also helps that you send in your measurements to them when ordering the armor), but for the higher levels of approval such as Expert Infantry and Centurion, I've had to make some adjustments.
A friend who is 164cm was too short to be a Stormtrooper and couldn't do a purchase from RS!