Does corrupting everyone pretty much lock you out of reliably getting higher tier units for the different elements?
I'm kind of hoping that upgrading elemental affinity at the alter would still allow the possibility of getting better unit types without matching two of the same type but my grasp of the mechanics is still pretty meh at the moment.
If my assumption is wrong... does that mean that corrupting should generally be avoided unless you want to use the person as a leader? The dark element seems good, but not good enough to miss out on most of the unit roster unless I'm missing something.
While the answer there is yes, nycto units are powerful enough that even if you had to forsake all others, you could still clear all current content with them. Lumine is also a source of non-nycto units, albeit so random that you will have trouble getting anyone specific.
A squad of nycto lawachurls with nycto archers will destroy whatever poor bastard is on the other end, no matter what you throw them at. Only Morax will give you any problems on hell difficulty, and even then the problem is more ganyu than morax himself anyway. Ziquiong is perhaps the one unit in the game who is immune to this strategy, but in her case, you can somewhat cheese her with unusual hilichurls anyway.
That said, you aren't wrong about hesitation on corrupting. Early game, you want to corrupt either Fischl or amber, but it is unwise to corrupt most girls due to early girls not having enough elemental energy to give you much more than archers. With that said, you should instantly corrupt Ping, Alice, and Xianyun the moment you get them, due to them having enough energy to reliably provide samachurls and lawachurls (though you are basically at endgame by the time you meet Xianyun anyway, at least in this version.)
Honestly, my recommendation is to corrupt Amber early on to get nyctos (Unless you mask her, she can't give you mitachurls, and you are better off saving that for Collie/Rosaria anyway due to shield mitachurls being important for holding the line reliably) after you get pyro slimes and at least one pyro hilichurl. Fischl is also a decent option for early nyctos. Do not corrupt collie at any cost, because your objective with her is to mask her as soon as possible in order to endlessly churn out dendro mitachurls.
DO NOT CORRUPT NOEL. Corrupting noel is a waste of her breeding capabilities because of geo not having a whole lot of reps that can easily be accessed, and post liyue, there aint gonna be a whole hell of a lot of them coming either, as I think the next female geo unit is
Navia in Fontaine? At least by order of release. So the three we have from mondstatd and Liyue are the only units we are going to have access to for a long while unless the dev gets all 'eye of the tiger' about rushing us to Fontaine.
Every other element is going to have tons of reps, however, so corrupt as you see fit for pretty much everyone else. Pyro units are basically free to corrupt for the time being due to pyro units not being all that useful outside the early game, since they have no lawachurl. Alice's elemental reserve is too useful for nycto to waste on breeding ordinary pyro units, so just keep hu tao or something on hand if you really want pyro units that badly.
The most important thing to do however, is to make absolutely sure you breed slimes of every element before you corrupt someone who can provide them. Slimes are very important due to resistances, and unlike hilichurls who you can somewhat work around with Special NPC trainers, they can't be made without a vision of the same kind. So if you plan to corrupt someone, try to get all the way to spectre if you can, and
then corrupt them. Electro slimes in particular are going to be important once we get the Inazuma release, due to the utter clusterfuck of electro units that we will be going up against (Yae, Sara,
Raiden, Shinobu...).
TLDR: Corrupting is a tactical decision like any other. Be sure you are ready to sacrifice their ability to breed powerful units of that element before you consign them to the nycto black hole.