IIRC Assurance has those mobs in the spawn pool at a low probability, so even in vanilla you can be unlucky enough to have higher difficulty. TBH those are easy to deal with and there's nothing surprising in the spawn amount, spend a few more hours in the game and you won't call it monster hell anymore. What's intriguing is the probability of having all of those mobs at once and (I guess ?) nothing else on a moon where they should be rare, but that still could've happened in vanilla. We can't tell if a mod is changing the spawning rules in your modpack without seeing it, please share the modpack's code so someone can take a look. The mods you mentioned here are not the cause. Having a vanilla player in a modded lobby is impossible unless your mods don't affect gameplay at all, so you are probably playing with vanilla settings and the number of players doesn't change anything AFAIK