I set up a special "mite" tag for the mites, which is what I think faoladh was asking for. Before I do any more mites, though, I wanted to talk more about how I see them surfacing in a game. The obvious way is to just have a swarm of them, either placed in a room or wandering random corridors, exactly like a standard monster. Another is as a flourish to describe a wizard's tower; perhaps there's a habitrail setup with a few fo each kind of mite. Perhaps even an eccentric noble may hire PCs to track down rare mites for a collection.
But I was thinking also of infestations in a PC's home base. Infestations would start small and grow if unchecked. This would start with 2d6 individual mites, all of one type, attracted by some superficial interest of their "class". An M-U might find ingredients are going missing; a Thief might find a coin or two on the floor, perhaps near a mousehole. Pets will suddenly growl or bark, acting as if they've seen some intruder.
As long as there are fewer mites than the PC's Intelligence or Wisdom (use higher score,) it's not a big deal. However, the infestation will increase 10% per week if nothing is done about it (just add 5 to the number of mites and drop the last digit.) Once a month, roll a reaction for the mites; on a Good reaction, the number doubles, while a Bad reaction means the number is halved (the PC may find tiny dead bodies, if the number lost is greater than the PC's Int.) For either Very Good or Very Bad reactions, another variety of mites has shown up, either to fight the original infestation or to join in on the bounty.
Once there are more mites than the PC's Intelligence score, there's a recurring risk that the mites will cause a bigger problem. Sorcerer mites can unroll a random scroll and trigger the magic within. Knights and elves start killing mice and leaving the bodies everywhere. Dwarf mites cause structural damage. Thief mites start stealing significant amounts of treasure. If there are more than one variety of mite present and food is not freely available, the PC may lose sleep from the noise of tiny battles. Roll 1d6 per 25 mites per month, with every 5+ on 1d6 indicating an incident.
And, of course, once the number of mites reaches 100 or more, they may actually swarm and cause serious damage.