As far as trimming...that is up to you. Personally I lollipop my gurlz and remove all foliage from under the canopy. It gets little or no light and will only suck developmental production from bud sites..IMHO.
As for flushing...many schools of thought there. Some never do it, some do it between cycles, some do it only at the end of veg and still others only at the end of flowering. Lots if threads on the subject...the search tool is your friend. In your particular case if you are going to flush...you could set the tubs on top of a grating of some kind...(an oven rack would work great). Place it over your bath tub...or outdoors if you can and flush away. Also if you are going to flush...the general consensus is to use 3 gallons of water per gallon of soil. Personally, I use about 1.5 gallon per.
But hey, get other opinions and make an informed choice. At the end of the day it is your grow.