As long as you meet the posted minimums for ATP you can take the checkride in whatever category and class you want. Of course, you're going to have to be highly proficient in the checkride aircraft to have any chance at passing. I took and passed an ATP multi ride holding a commercial single with no multi priveleges at all.
Please pardon me if I am wrong.
To have an ATP add-on (eg. AMES), the applicant needs to have minimunm 50hours in a multi-engine seaplane in order to have him rated for that AMES class right?