a 300 gallon tank is going to be quite the undertaking. When you say the first thing you need to do is get a job... that's an understatement! lol
you can certainly "mix" the
LS with the southdown, but what I would, is layer the southdown on the bottom, then add the
LS on top. don't worry about mixing.
if it doesn't have built in overflows, consider adding some. What do you think you'll do about a sump for it? (thats what the overflows are for anywya)
and for circulation, a number of powerheads is one option, I would research and plan out a good closed loop system.
IMO that will be much more efficient. and a lot less "stuff" in the tank.
and of course the test kits and water quality equipment mentioned above are a must!
IMO, get a refractometer, rather than a hydrometer, just my personal preference.