I think the sand fleas / mole crabs should be livestock critters or a non-issue. Those are probably a great critter to roast and eat once large enough, but I doubt those create a confrontation with mice or cause a competition for resources. Regardless, an interesting critter to describe.
For bubble crabs, I might treat that in the same way. It’s great to hunt and roast for meals, but they don’t really compete or confront mice. The method of gathering food just does not create a strong sense of being a threat to mice. As far as I understand the bubble crab anyway.
Horseshoe crab maybe a little different, but my understanding of them does not present a threat to mice as a predation threat. I imagine the large size would make them seem much more imposing, and I’d use that as a format for frightening those who are unwilling to pause and think it through calmly. So, if there is a trait that fits, and a proper space for a Twist, maybe present the horseshoe crab or two, and explain that the trait creates a frightened impression–not the Angry condition, just an impression of fright–and see what the patrol does. maybe they ignore it and wander away.
I treat spiders as livestock critters. I actually just published a livestock critter pack on Prevail which includes the orbweaver spider. I imagine that spiders are useful and could be eaten. A bit of Insectrist training could garner a garden or field defender or other possible supporter.