There are a lot of live plants that you can use in a terrarium that will not have any affect on your allergies. Go to a plant nursery in your area and an educated sales person there should be able to point you in the right direction. That way you could see the plants and make sure they would be the proper size for your setup.
i have horrible allergies too so i understand that lol but i have live plants in mine that dont bother my allergies at all, so there are plants out there that you could use