Philip's advice is very sound. There are so many variables and so many things can go wrong, making it difficult to pinpoint the source of problems. When I started out I transfered onto fixed out glossy and matte fibre photo paper. The transfers just glided on nicely. Tonally, very nice. But it's been art papers ever since. I don't feel right fixing out photo papers... that's me, though.
To answer your question, I size only with acrylic medium. No gelatin required. I used to zap the sized paper in my drymount press, but stopped doing so due to weird sheen from the texture of the paper. It was lost in really dark areas, but lighter areas it was clearly visible and very annoying. So now I put a few drops of ammonium dichromate (6%) in the acrylic medium for each coat (4 coats). Then I zap it with UV light for 10 minutes.
I've been using albumen over gelatin more and more these past few months. If I want more sheen, I put a few more coats of albumen on. Transfers are very nice, and this combination seems to hold slightly more detail in the high values.
Some papers are waiting at home for some albumen... stupid staff meeting!!