Like materials, biomes do not blend, but are pooled by the grammar until they are "picked up" by an appropriate primary symbol. Single Biome Controller will only take a single biome, and Native Biome Controller takes none, but the other Biome Controllers can absorb any number of prior biomes. ach Floating Islands symbol takes a single biome. As of Mystcraft v11, if biome symbols are left "dangling" at the end of the book (and a Biome Controller has already been specified), Floating Islands symbols will be supplied to absorb them.
All vanilla biomes are represented by symbols, as are any biomes supplied by other mods, unless the mod has taken steps to suppress their symbol. This means that some biome symbols may be available that represent work in progress, or which were not intended to appear as worldgen biomes. Using such biomes in an Age is fairly hazardous, ranging from unreasonable or dangerous contents for the Age, to actually crashing the game. A couple of examples from Thaumcraft include its Eldritch biome, which generates as bare dirt, but spawns the advanced monsters found in Thaumcraft's Outer Lands (an endgame dimension). A more threatening example is currently found in the Thaumcraft add-on Thaumic Horizons, whose Pocket Plane dimension will generate with a surface made of barrier blocks which don't render properly and cause large amounts of damage on contact when used in an Age.
The Nether's Hell biome has a symbol, which when used in an Age will spawn Nether mobs, but will not have the Nether's special prohibition on water, nor will beds be explosive. The End's Sky biome likewise has a symbol, but its page is restricted to Creative mode because Ender Dragons can spawn in any Sky biome at coordinates (0, 0) regardless of the dimension.
Biomes can be disabled individually by disabling symbols in the config file; the corresponding symbol is the word "biome" followed by the biome's identification number. This means that they can and will vary from modpack to modpack.