Well, unfortunately cutting 'unnecessary' items is a subjective... art. Yeah, that's the most appropriate word. I'll post my recommendations and reasoning:
1. You only need to see the top half of each HUD. If the "top" of your capacitor is off-screen then you're in peak recharge. If you see it on the left, you're below peak recharge and you're in trouble. That saves a lot of space.
2. You don't need to see the entire overview of each character. As I explained in my video (shameless plug:
https://www.youtube.com/watch?v=peBPbR-sPck), as long as you sort by icon then everything will always be in the exact same order. Always. If you want to see an individual character's overview to do something specific, just swap to that toon. The amount of space that displaying all your overviews uses compared to what you lose by not being able to see them all is ridiculous. It's too much information and takes up too much space.
3. Once you've fixed your overview, you don't need to display your individual targets for your DPS characters at all. Just cut'em out. You can target from your overview.
4. You don't actually need to group a character's modules near the rest of their stuff, you can just put them in neat columns or rows. Is it not flashing green? Then turn it on. This saves on how much of your screen you have to look over in order to figure out if a character's modules have cycled off because they got neuted or jammed etc.
Multiboxing, especially in EvE online, is about paring down how much information you have to have access to, and then taking that information and making it as efficient as possible. For instance, viewing each character's overview is nigh on worthless, since the amount of time it takes you to find the correct overview and assess what's going on and then make a useful decision is just too long. Active hardeners, Sebos, Links, etc should always be on--You don't need to know *which* character's shit turned off, just that it's *off*.
In any case, go watch my video (second shameless plug). I don't know how to make my setup more efficient space-wise. It's as compact as it can be. For less characters, it would take up proportionately less space.