Bonestorm Hatchlings are difficult to counter somewhat, due to the likelihood that they will get Bonestorm off at least twice in a fight, thx to the Heal and undead racial. One round stuns work well for the undead racial round, and Sandstorm actually works pretty well at soaking some of the dmg from all three of your pets. I would say that a Silithid Hatchling
running with Scratch and Sandstorm would be the likely best overall counter to the F. Hatchling. A DAH that uses BP on the round that BS is likely to be used is also effective, unless juked.
As to the Idol, he is a bit trickier in that he is good against practically everything, and is even decent vs. undead because of his high health, defensive abilities, and big dmg output. In all honesty, Creeping Fungus teams won't work too well against an Idol team because he will just switch the weather on you, and SS will take most of the dmg from an unbuffed CF. CF is further compounded by the fact that no pet who can cast it comes equipped with Moonlight, meaning at least one switch, maybe even two. If memory serves, Moonlight only adds 40 dmg per tick, equaling 160 dmg over the course of the four round DoT, which is not worth the lost turn for the switch IMO unless it gets applied on at least two pets for it's duration. I run a few Moonlight teams, and CF becomes more viable if you have another pet that can also take advantage of the weather, such as Xu-Fu.
For Idol killing purposes, I have a few pets that I prefer, even though they don't seem ideal. The first is the Warbot. He can generally go toe-to-toe with the Idol thx to Extra-Plating, and has the added perk of putting down a Minefield to score a big hit on the next incoming pet, which will mean that the Idol player will usually leave him in front in hopes of out-waiting the MF. While the Warbot will miss a few shots due to low accuracy with SS running, he hits hard enough to normally either kill the Idol, or seriously hurt him making him easy pickings for the next pet of yours. As stated before, Mr. Wiggles also works well, though he generally takes two rounds to buff up to be effective (Crouch and Uncanny Luck, then attacking). I also really like using the P/P Twilight Fiendling to put the hurt on Idols. Adrenals Glands coupled with Creepy Chomp will destroy an Idol quickly, thx to the crit buff. Blighthawks are also very effective at killing Humanoids, with big dmg potential.
Now, if you just want one pet that can counter both Undead and Humanoids, that is a much trickier proposition. I would say that my favorite to handle both would probably be the rarely-seen Stone Armadillo
, Running with Scratch/Shell Shield/Infected Claw, he can do big dmg to both pet types. SS is good for soaking Haunts, and makes him very durable vs. Sandstorm pets as he will then have two mitigation shields going. I have the H/P version, but in the case of F. Hatchling warfare, I would say that the H/S version may serve you better, but that is debatable. I mostly use him against valk spam, so the speed isn't as much an issue and I like the bigger hits. The SA has another added benefit as well; being a critter, he is great vs. Lil' Rag and the QG! No stuns from the trap or the kick. Watch out for DAH's though, he will get wrecked by them in no-time.