2 FX5s filled with bio will be plenty for biological. I have no experience with those other canisters you're talking about for mechanical, as I use HOBs for that, but that's not really an option for you due to the hood.
As for the bio media, in a large canister filter I would recommend seachem pond matrix. Firstly, it's bigger than matrix, so fits in my FX6 without the need for any media bags. This is very important as small pieces can go into the impeller at the bottom. Secondly, due to the large design of the stones, there is meant to be an anaerobic zone in the middle where DENITRIFICATION can occur (reduction of nitrates). This really only happens in the anaerobic confines of a canister filter, and the slower the flow, the more beneficial.
I would stay away from all bio noodle type media. The surface area and denitrification just aren't there, plus many of them break down over time into dust particles which can jam the impeller and you have to replace the media. Matrix never degrades. Just my 2c. If you're worried about cost, buy a 20L (5G) bucket of pond matrix online or 2 x 4L (1G) buckets., which is costs almost as much. Much cheaper in bulk.