answer:I don’t know of any spiders that are “vegetarian”. That is, I don’t know of any spiders that will harm your plants directly by eating any part of them. (If I’m wrong about that I’d love to learn something new.) As for mites and other insects, if your compost still has those running through it, then it isn’t yet decomposed enough to be ready for use as planting / bedding material. Let it decay some more, with or without the bugs. When there are no more visible insects (worms are to be expected, but not insects), then it may be more suitable for use with planting.