Hiya,
The direction you have the filter flowing is up to you, thing is though, as a rule you get more circulation with it aiming down the longest way
I have 2 externals in a 6x2x2 As you look at the front of the tank, rear right i have one aiming from the rear right corner towards the centre fron of the tank
That gets the water circulating nicely across 2/3rds of the front
Now the other is rear far left, this just faces straight down the back towards the other filter, i find aiming one across almost 45 degrees helps maintain a decent flow
To cut it short, if i were you i would aim along the longest length but also try aiming the flow to deflect n bounce off the opposing end (imagine snooker shots bouncing of the cushions) til you feel you have the flow you want/need
As for the substrate, sorry i have no idea, though i use sand myself, find it easyish to clean regular, just slow syphon and steady hands lol
TTFN
Tony