Yeah you can get a 1st class medical with allergies; and I've even got asthma to boot! I've got the regular seasonal (spring/fall mostly) sinus infection/allergies kinda crap going on, and I've got major cat allergies also. I'm on singulair for my asthma, and it's also being perscribed for allergies these days. If they don't like what you're on (and I'd check the AOPA list before you apply for your medical), then try to get on something similar that will work for you.
Oh, and if you're holding down a 3rd class right now; the 1st class is no different in the way the FAA treats this stuff. Either you can get them all, or they'll deny you outright for any medical.