yeah, I think you might be overthinking it. The demons themselves aren't really that important. You want to think in terms of
1.) Skills
and
2.) Resistances.
Skills are going to be elemental spells in the vein of zio (electricity), agi (fire), garu (wind), bufu (ice), hama (light), and mudo (death). a given skill has different strength levels (Bufu becomes Bufula, Bufudyne, and eventually Niflheim) as well as multi target ability (mabufu, mabufula, mabufudyne). There are physical skills also, but those are less important since anything weak to phys can simply be hit with melee.
the important thing is to have a good mix of skills, since the press-turn system means hitting an enemy weak point means another shot at them. Find someone weak to wind? switch to someone who has garu or garula and not only dominate them in terms of damage, but get free turns. The "level" of the demon is secondary. There's not really a good need to start fusing your current demons (even if you can) unless your current lineup isn't getting the job done. At that point, look at what you have and see if you can possibly get the skills you need through fusion.
Resistances are the opposite of the above- a given demon might be weak to ice, but strong to fire, or strong to fire and weak to wind, or strong against everything BUT hama. If you come across a demon that slings fire spells like they're on sale, switch to one strong against those spells or at least try not to use one that's horrendously weak to fire spells. Again, press turn means that getting hit with a weakness means you're giving up a free hit.
There's never any reason to get attached to a specific demon, or spend a bunch of money re-buying them unless there's a VERY specific fusion you're trying to hit. A fused demon will always be stronger than the one you had, though it's skills might be different. think about the results you're getting before you hit the "ok" button when fusing. sometimes giving up your only demon strong to death spells might not be worth it if there's a lot of enemies slinging mudo around.