Note that vanilla Mushroom Island biomes are as resistant to hostile mobs as the Rainbow Forest. Getting one tainted seems unlikely as well, because they're... well... islands.
I'm fairly sure the anti-taint feature of the rainbow forest can be configured, but I'd be surprised if the hostile mob thing was configurable.
Edit: Ninja'd. Blarg.
Mushroom forests are rather small and can't be expanded. I've got a couple pretty large rainbow forests on the new worldgen and something in the config file indicates you can expand them by planting rainbow trees.
It is tied into the nature of the biome.
I think I'm just surprised that I didn't see that in the changelog or the descriptions I have seen. Given the kind of hard servers I run, I'd have given serious consideration to not spawning mushroom forests because they are safe havens. So now I have a second biome that I would consider removing in worldgen because it has a significant impact on the server theme, but that's contrary to your mod license and could have an unknown impact to progression in the mod.
Thaumcraft's taint biomes by comparison are a negative biome and are configurable. Would hard mode in Chromaticraft justify the forests not being safe either?
In the interest of full disclosure, part of the reason this concerns me is that on our new server, as deadly as it is for all players involved, my wife and I have settled at the border of a magic forest (Thaumcraft) and a Rainbow forest. Both of us though it was a great spot and had quite a view. If the rainbow forest is perfectly safe against Lycanite's monsters, imagine the implied meaning of the server admin living in a safe zone. Whether I knew it or not when I settled.