Two questions:
1) How do I get liquid enderium - I have it in ingot form, but it can't be put in magma crucible and I don't know how to put it into a bucket via smeltry (needed for big reactors coolant purposes)
2) Do pink slimes die naturally? I made a pit and left 4 smallest-tier pink slimes in their, I came back they were all dead... any idea why?
As for #2, pink slimes probably despawn like all other non-passive mobs. Your best bet is probably to spawn some and capture at least one in a Safari Net, which will preserve it indefinitely and also let you spawn more from an Auto-Spawner if you wish.
Another tip, you can get a lot more slimeballs out of one slime if you Embiggen it first- the Slime Embiggening Syringe costs only a couple of slimeballs (and some lapis) to craft, and increases the slime's size by 1, to a maximum of one size larger than the largest naturally-spawned size (for green slimes, at least). It pays for itself quickly if you then kill the slime with anything other than a Grinder (which won't cause it to split).
If you use ExtraUtils Spikes or something of that nature to split your auto-spawned giant slimes to tiny size, then Grind the tiny slimes, you can actually get a net gain of Mob Essence. I don't know it it's enough of a gain to also power all machines involved with Reactant Dynamos, though.