I would bring the micron filter as close to the top of the canister as possible. Ideally it would be perfect if the cartridge was the entire lenth of the canister, but not much you can do there....Then I wouldn't use the stock fluval pads but use some 20 ppi or 30 ppi pads for your prefilter section to prolong the lifespan of the micron filter.
The only problem I see after that is will the fluval pump have enough umph to draw the water thru the filter. Typically these open impeller pumps have more power at the outflow then they do suction on the intake.
"I would bring the micron filter as close to the top of the canister as possible":
Well the cartridge is just shy of 10" and the contact point to the intake is just over 12", also the intake is off center by 3/4 of an inch in both right and left of center, deliberately I think to prevent someone from attempting this. So the 2.5 difference in height between the cart and intake helped make the correction using a small piece of 5/8" corrugated FX5 hose. The mating was done via a 3/4" threaded PVC adapter PVC-glued right over the intake opening to the impeller cover and the other end to a 3/4" threaded PVC pipe which is inserted snugly into the top of the cart opening. So every bit of space was used up yet the assembly (hose & threaded pipe) disassemble for cleaning easily.
"use the stock fluval pads but use some 20 ppi or 30 ppi pads for your prefilter section to prolong the lifespan of the micron filter":
The stock sponge insert assembly although a good idea was deliberately designed with way too much bypass, along the top, sides, front and back allowing water to enter the basket side right from the in-flow port and take advantage of all the media basket bypass water channels allowing as much as 60% of the water to simply pass from the inflow port to out-flow port without filtration.
The remedy was the bonding of 6 AC70 sponge filters in 3 rows of 2 sponges side by side lengthwise which provided a bock of sponge which was trimmer for a custom tight fit yet provided the channel necessary for water to be carried down the entire length of the sponge and canister or 100 SI of surface area, large enough to filter without clogging and no bypass.
"Typically these open impeller pumps have more power at the outflow then they do suction on the intake."
Theoretically becasue there is so much more filter surface area especially on the cart about 200 SI, restriction will be very low as if there in no media at all, so I am hopeful and anticipate that when it is ready for trials, (tomorrow maybe) it will work as well as it is put together.
Although the end result is a simple set of parts, it took allot of time and patience to work out the problem details. The most difficulty the reinforcement of the cart against compression from the lid and base, and the coupling of the cart to the impeller cover which had to be both strong and yet easily dissembled for cleaning.
Im still working out the final details and will post photos as soon as I get more feedback as I want to make sure I didn't miss anything so feel free to grill, ask and constructively criticize, and I try and answer every question.