As I was exploring Gehennom, I lost a couple of balrogs and vrocks when I fell through a trapdoor and couldn't get back to them in time to prevent them from going feral. I've yet to find polycontrol of any sort, so I'm unable to become a demon to retame. They are all peaceful, but the weakest is still 160 hp, and all are well-equipped (making them formidable, yet killable). I don't particularly need them anymore, so I'm wondering if I just leave them be, will they stay peaceful (particularly, after I get the Amulet and have to pass their level with Rodney on my tail)?
If your former pets have gone feral but remained peaceful, then they'll stay that way as long as you don't attack them (deliberately or by accident). Of course, they may still block your path, or get in the way of your ranged attacks, and they can attack you (even while remaining nominally peaceful) if you're wearing a ring of conflict.
The behavior of abandoned pets will depend on their tameness. Basically, a pet left on another level will lose one point of tameness after 75 turns, and every 150 turns thereafter. If this would reduce its tameness to zero or below, then it will stop being your pet when you next enter the level it's on, and it will be either peaceful or hostile depending on how long it was abandoned.*
The exact chance of the former pet being peaceful, as opposed to hostile, is (former tameness) / (total tameness lost while you were away). However, once the former pet has gone feral, it's just another monster, and no further tameness checks will be applied to it. So if it's peaceful now, it will not become hostile on its own.
*) As a special case, if the abandoned pet would've starved to death while you were away, it will turn hostile instead. There's a comment in the source code saying that this is to avoid such pets instantly dying of hunger on their next move. However, this check is made after the tameness check described above, so it only applies to pets that still have positive tameness left. Thus, a bit paradoxically, a starving ex-pet with (at least) one point of tameness left will turn hostile when you return to its level, but leaving the same pet alone long enough for its tameness to drop to zero will cause it to turn peaceful instead.