>>715314208>They slot in as being pretty much the same difficulty as vanilla bosses at the same levels.From what year? 2011? The only one that's really of an appropriate difficulty is the Grand Thunderbird, which you can pretty confidently call a King Slime-level boss. Everything else floats around Eye of Ctlulhu level, at best, until you get to the Star Scouter which drops one of the most broken sentries in the game.
Afterwards, you enter Hardmode and you need to go from the Fallen Beholder and skip all the way up to The Primordials since everything else inbetween has behaviors and patterns that make EoC look hard by comparison. You could maybe include the Forgotten One in there as well, but as long as you construct a rudimentary arena (dig a big hole in the ground), it's just a weaker version of Plantera.
You don't need to hold them to Calamity's standard. You need to hold them to Vanilla's standard. Most of the Hardmode bosses it adds don't compare to Queen Slime (the boss that's recommended to clear as soon as you enter HM) and most of the pre-HM bosses don't compare to Queen Bee or Deerclops.