Skimmers
Check the venturi for blockages, it may have a calcified build up, rouge snail etc, if it has a build up soak in vinegar overnight, rinse with ro and put it back in place if all looks well.
As there is no surface skimming, removing the oilly slick that you will see on the waters surface after you turn the power heads off will remove alot of waste.
Flow
Yes there is a thing as much flow, it cannot be assessed on a X per hour basis, it needs to be judged on visual observation, if things are looking happy as the tissue isnt being torn on the LPS etc then I would hazard to say you havnt hit that thought.
Sandbed
Your flow will shape your sand bed, I am personally against psydo deep sand beds in this style of set up but I will keep my mouth shut here.
Maintaince dosing
Save your money for the time being and do lots of water changes (read 20% per week) keep an eye on you salinty and when you visit your LFS get them to test your water. Once you are more confortable and have done a bit more reading, purchase high quaility test kits and assess your water yourself. establish base line parameters, your desired parameters and build a structured method to achieve it.
But I really dont think you will be achieving anything out of this set up that 20% a week water changes wont keep in check.