I've had this problem, and it's all down to the new Biome System in Minecraft.
Pre 1.7 you had 256 ID ranges for biomes.
In 1.7 all IDs above 128 are variations of the biomes below 128, so Mojang have in effect halfed the number of free biome slots.
Add in Biomes O Plenty (or other biome mod) and that'll use up most of the free slots, then many other mods use biomes too (Thaumcraft, Buildcraft, etc)
The only option is to ensure that the TF biomes that can spawn in overworld are the more "normal" ones and match the overworld counterparts. Took me a long time in my modpack, but the chances of other dimension biomes appearing in overworld are very low now, and they do fit quite nicely. You don't appear to get the unique dungeons spawning, so it's not too much of an issue.
If you want, checkout my public pack (OmegaDawn) to see how I've done my IDs. I also use The Erebus, alongside TF and many other mods adding biomes, so it's been a juggling act.
Biome IDs are pretty much the only remaining ID conflict left (along with potion IDs).