I've run a server with aternos for a friend and I and noticed a few issues with mob spawns especially with modded creatures. I thought I could use the kill command to target the specific mobs that were over-spawning (despite a mob cap being in place) and eliminate them without murdering everything else in the world, however, it seems like sometimes the mob selected does not die and everything else in the server dies including tamed creatures and vanilla things. Or, the second option happens where the target creature dies including everything else despite me using the tags for the mobs provided in the auto fill. I've tried a few different variations of the same command prompt but it seems as if selected modded creatures doesn't work even when I use the !#alexsmobs:(x) portion for the target selection.
I've noticed this issue with mobs like hummingbirds and especially the seagulls. The gulls don't die unless I end the command with /kill @e and leave it like that, killing everything.
Example: "/kill @e[type=!alexmobs:crow]
doesn't work for just the crows. It kills all mobs.

is there something I am doing wrong in game? Are the type tags not correctly represented in game? Or is this an inherent issue with the mod currently. My version is java, forge, 1.20.1.

You are inverting the type there (saying: everything except this mob type)
remove the ! in the command.