I like your skill choices. Diplomacy might be worthwhile as well, since they can be skilled merchants.
For feats, how about Skill Focus (any two Craft skills)?
LA:
+1 for unbalanced ability scores
+1 for natural armor
+1 for having at-will spell-like abilities
+1 for having spell-like abilities beyond a caster of comparable level
+1 for spell resistance
+1 for two types of energy resistance
---------------------------------------
+6 Total
Weight...200 lbs.?
A plumach can be summoned using a summon monster (4 or 5?) spell. (It is better than a quasit at summon monster 4, but worse than a bearded devil at summon monster 5).