Trickery gives you Bluff, Disguise and Hide as class skills, which you definitely want as a Doppelganger. Make sure you have decent INT, so that you can put some skill points into these as well as Concentration and one or two other skills (i.e. INT 14 or INT 16).
Evil seems a more powerful domain than Knowledge for an evil Cleric. Knowledge gives you some more skills, but you're already going to be short on skill points, as they're going into Bluff, Diguise and Hide ... so no need for more skills.
I assume you mean a Doppelganger (4 HD + 4 Level adjustment)/Half-Fiend (+4 Level adjustment)/Cleric 1. Do you realise that your character is extremely weak compared to a normal character of that level? Doppelganger with class levels is already weak, but better than normal with Disguise and Bluff. Add in Half-Fiend, and you really don't have a leg to stand on. You'll only have 5 HD total at 13th level ... so you'll be getting damage reduction 5/magic, spell-resistance 15, Darkness 3/day, Desecrate 1/day, Unholy Blight 1/day from Half-Fiend ... it's not really worth it unless you have a high class level.
Hrmmm and the Evil domain and Half-Fiend abilities overlap a fair bit, so Knowledge might be a better choice for versatility.
Just my thoughts ...