Looking for monsters for a dungeon

[Aleolus]

I am building a dungeon for my party, and am looking for monsters that would plausibly inhabit it. It is a massive, abandoned iron mine (abandoned for ages and ages). Massive chambers, long tunnels between them, a few cave-ins here and there, no manufactured traps (maybe some pitfalls or other naturally occuring ones), and so on. Rust monsters are a definite (the party is coming here specifically because of them), but what else would be appropriate?

[MesiDoomstalker]

What CR range are you looking for? That is a very important factor before we start populating a dungeon. You mentioned Rust Monsters, so I'm assuming mid to low CR; nothing about 8.

[Biffoniacus_Furiou]

Giant spiders and giant centipedes would be fitting, the ecology could likely support them.

This stuff (https://www.d20srd.org/srd/dungeons.htm#slimesMoldsAndFungi) can survive anywhere.

[Mike Miller]

Balhannoth

...


More seriously, carrion crawler near the entrance. Grells in vertical shafts... Grimlocks, maybe being led by something scarier than them

[Saintheart]

Shadows. Maybe allips. Lots of unquiet spirits from cave-ins down there.

[Blue Jay]

If you're looking for stuff around the rust monster's CR 3, here are some ideas:
A colony of giant ants (https://www.d20srd.org/srd/monsters/giantAnt.htm) would make a lot of sense down there.
An ethereal filcher (https://www.d20srd.org/srd/monsters/etherealFilcher.htm) might squabble with the rust monsters over metal treasures.
Some derro (https://www.d20srd.org/srd/monsters/derro.htm) and/or duergar (https://www.d20srd.org/srd/monsters/dwarf.htm#duergar) could be exploring the mine.
A gelatinous cube (https://www.d20srd.org/srd/monsters/ooze.htm#gelatinousCube) with loot floating inside it is kind of classic.
A grick (https://www.d20srd.org/srd/monsters/grick.htm) makes a nice random monster, or the pet of a CR ~4 spellcaster.
Maybe a shadow (https://www.d20srd.org/srd/monsters/shadow.htm), if your party has ways of fighting an incorporeal creature.
I've always found thoqqua (https://www.d20srd.org/srd/monsters/thoqqua.htm) encounters kind of fun to run as a DM, as long as you're not too annoying about making them stay out of reach.

If you're looking for something bigger, maybe some of these would be thematic:
A serpentir, CR 4 (from Monster Manual V, listed with the Skull Lord), seems like something scary that might be lurking in a dark mine.
I think the rast (https://www.d20srd.org/srd/monsters/rast.htm) (CR 5) is underused: it was the first monster I ever sent against an online party. Good for a wandering monster
An avalancher, CR 5 (form Monster Manual III), can cause cave-ins and manipulate the stone around it.
A tomb spider, CR 6 (from Monster Manual IV), along with web mummies and broodswarms as wandering monsters.
A creeping stone, CR 6 (from Dragon #314), basically an ochre jelly, but with Earth Glide and an engulf+petrify attack


If you're looking at even higher CRs, here's a couple that could sort of replace a rust monster at CR 9:
A lodestone marauder (from Monster Manual IV) disarms characters with magnetic forces
A blade spirit (from Return to the Temple of Elemental Evil) can animate unattended weapons

[ShurikVch]

Ghosts of lost miners?

Xorn (https://www.d20srd.org/srd/monsters/xorn.htm)?

Lava Ooze (Sandstorm)?

Iron Dragon (Dragon#356)?

[Aleolus]

Levelwise, I'm expecting the party to be about 5 or 6, but I am looking for the area to be around 4 or so. I don't want it too challenging for the group, especially because this is going to be a massive honeycomb underground. (This mine is actually going to be an aqueduct in a city for a different game I am doing set a long ways in the future).

Also, the Rust Monster is needed because the group is going here specifically to collect one of their antenna for a spell component