The topic is on the FTB Ultimate's version, there are quite a few fixes down the road so yes, it is not supposed to be sustainable just by itself.
MFR only, you will need to run Breeder, Chronotyper (maybe) and Grinder, running on farm animals for your mob essence needs.
But since you are running on the Unleashed packs, there's a few ways you can go.
1. Regular spawners picked up using Force Wrench, make a regular spawn room out of them and use grinder to kill them for loot and essence.
2. Soul shards. Start out, make a tier 1 or 2 shard out of the more common mobs like skeletons, zombies or spiders. Either by manual killing or using the shard on a spawner. Using shard on the spawners will destroy the spawner and yield 200 souls for that particular soul shard.
Then, utilising the low tier soul shard, make a mob farm using Grinder to get your mob essence.
3. Extra Utilities. Sigil activation ritual, cursed earth. Very long winded, Google it. Sorry.
There might be other ways too but I think the above should be the most popular and easiest method.
Once you got the mob essence flowing, it's time to go use your pokeba*coughs* I mean, Safari Net (Reusable), to catch 'em all!
P.S. Auto-Spawner mobs can be killed for souls to level your soul shards, so if you did use lower tier shards earlier, you can now upgrade the essence farm to a t5 if you want to.
As for your question regarding enderman. Yes, you can use spawn them using the Auto-Spawner. Though I recommend making the room 9 by 9 with 6 height. But that is just me.