What would you put in the 5.5g FW tanks? I've seen some neat shrimp displays at one of my LFS. I had no idea there were so many species of FW shrimp available these days.
Shrimp are high on my list of considerations! I'd like to get a few different types.
I also like some of the tiny fish such as celestial pearl danios, mosquitofish, dwarf rasboras, least rasboras, chili rasboras, blue rice killifish, maybe some guppies/Endler's. Some of these might need a 10g as noted below, since 5g might work, but may be less than ideal for the long-term.
One or two might end up as betta tanks.
I've also got plans for linked-plumbing racks of 10g tanks for smaller ecosystems including occupants that get too large for a 5.5 but stay small enough for a 10.
These are lower on my priority list right now, though, which is to get my main SW 125-125 system up and running after finishing my living room remodel; changing the 75-75 QT's over to FOWLR with butterflyfish and snowflake eels as primary choice fish; setting up my 150 for goldfish, hillstream loaches, white cloud mountain minnows, and a few other cold-water options; and changing over some of the FW 40B's to SW and finishing the 2X4 double-stack 40B systems.
My plans will already keep me busy for a while, let alone the endless dreaming! Patience and proper research are most important.