I'm a supporter of enclosed emitters too. My personal choice is an ACRO P-2, but the Holosun 509T has been run by at least a couple of guys locally with no problems, one red and one green if I recall correctly. I believe Utah Highway Patrol are running them on Glocks. The 509s have a crossbar mount "borrowed" in concept from Aimpoint. I have it run the full size EPS, but have run the smaller carry on a G43X, no issues. I think folks see the crossbar design as more secure.
Holsters: for concealment for me, JMCK IWB3 for a single cell Streamlight. They make an OWB too.
The X300 is a big light relatively speaking. I have a Tenicor OWB for a Streamlight TLR7 (one cell) I find quite comfortable; they make one for X300s. No retention except friction on any of these. Safariland does make an ALS that's not completely out of the question w/retention.
I'll echo what others have said: mounting is critical. @
SoCalDep did us all tremendous service in working out a procedure for the mounting of plate and optic. Follow his work carefully.
The other piece is instruction/training/practice-get some and keep it up. Gunsite(@Erick Gelhaus teaches there) has an immersive program. Manufacturers-Aimpoint, Glock, I think* Trijicon, offer classes and there are private individuals.
You can go it alone, but a good class/instructor will shorten your learning curve.
Best of luck