This works for all Gaia versions!
Can I use multiple biomes? The answer is yes!
You can create multiple biomes or use multiple biomes presets- you are not limited to just one!
When you first create your world in the Gaia Manager you have the option to add a biome:
To add another biome or preset biome you simply can go to the advanced tab of the Gaia Manager - Gaia Tools - Add an existing biome:
Please note that you are still limited to textres per terrain:
So keep in mind not to just spawn the textures (if the biome has a texture spawner) and start spawning. Take a look through the biome and use what you want out of it!