Any plan that involves taking packs apart and testing the pieces, then reassembling them, is useless for grid storage. You see it in the "DIY Powerwall" communities, when a decent number of the cells coming in are quite literally junk (fully discharged, shorted, etc), but anything like that particular group does, at scale, is a battery warehouse fire waiting for a time to happen.

There's actually not as much variability as you might be inclined to think - in terms of pack voltages. China is their own set of problems I'm not familiar with, but in the rest of the world, most modern EVs are using a 3.7V nominal chemistry (2.5V/cell fully discharged, 4.2V/cell fully charged), and almost always in a 96S pack of some capacity or another (so about 400V fully charged, again, give or take a bit).

If you're dealing with 3.7V cells in 96S packs, the rest of the pack details simply don't matter beyond individual pack management and balancing. The voltages of the storage rail are compatible across all of them. Some may be 50Ah packs off a PHEV, some may be 250Ah packs off a long range BEV, but they all operate at the same general range of voltages, and so can be put on the same voltage rail once they match.

You'd want per pack fusing (probably a contactor assembly that can report current, and disconnect a misbehaving pack if the current goes too high), but at that point, if you can talk to the BMS on each of the packs, they'll just work.

The only real gotcha with this is getting the pack voltages lined up with the main voltage rail in the first place. If the rail is sitting up at 390V (almost fully charged) and you put a 250V pack (fully discharged and then some) on it, the internal resistance isn't enough to prevent you from pushing an awful lot of amps into the pack and likely damaging something. So you'd have to have a separate set of chargers to get the packs close to the main rail voltage, but once it's within 5-10V, hook it up and it will equalize out safely.

If the facility is somewhat climate controlled, you likely don't need to hook up the cooling loops for the packs (if they have them) - but you might want to, because the difference between a failed cell and a runaway pack is typically going to be the cooling loop. If a cell fails and runs away, a pack with good coolant flow probably won't propagate. You might not want to be near it, but... it probably won't burn the facility down. Without cooling flow, welp. Whole pack will just run away, one cell at a time, and that's the sort of thing you want to watch from a very safe distance.

The practical limit on pack life is going to be how evenly matched the cell groups are. If the pack is worn evenly, all the cell groups will generaly decrease in voltage equally. But as the pack wears out, if one cell group loses capacity more than others (it may be a hot spot in the pack), you start having to cut off the discharge as that group hits the low voltage cutoff (while the bulk pack voltage is still acceptable). If this starts tripping the pack off too early, it's not going to be useful on the rail, and it's probably time to recycle it.

Really, though, it comes back to how much risk you're willing to tolerate. An individual car battery pack is likely to be perfectly fine. A warehouse of 5000 of them, out of used/wrecked/who-knows-what cars... that's a little bit scary. If one of them runs away and isn't contained, you have a warehouse full of packs running away, venting violently toxic chemicals where there's no good option. If the vent plumes are not on fire, you stand a good chance of wonderful things like HF (there are less friendly things to humans, but not many). If the vent plumes are on fire, the resulting gasses may be mildly less toxic, but now you have what amounts to a giant hazmat warehouse fire.

I'm really not sure what the right balance is, but with the challenges and risks of second life EV batteries, I honestly don't expect them to be particularly popular with any of the big energy companies.