This is a subject that is just starting to confuse me too, so far I have bought quite a few of the great little 2hp modules and it does give quite a bit of constraint for fingers and patching. So I'm now thinking about Layout, all the small 2hp modules want to be scattered around so they are more accessible around lager hp modules.
I'm basically working across 3 rows, and I'm thinking i want to try and have the control bits like modulators, sequencers and Mixing on the bottom row then push the sound sources up and maybe the effects too.
I'm hoping things just become apparent where they should go as I buy more modules!!!
I'm half tempted to get another case and purpose it just for the output sections of mixer, effects etc etc
Good Luck!
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.