Kain Darkwind
Explorer
Regarding the summon chance. While the older editions had a failure chance, should our version? Pit fiends are supposedly weaker than Dukes of Hell, but they have automatic success. Lords of Hell are obviously more powerful than Dukes, and they have automatic success. The obvious medium between the two is....automatic chance of success. 
Moving on from there, Abishai are all low low CR. Under 10, if I remember correctly. It wouldn't be out of line to let him summon 2d6 to 3d6 of them, if that is the only devil we are going to let him summon. I think all dukes should be at least capable of summoning something CR 13 or better, as pit fiends can summon a horned devil 2/day.
As a possibility, perhaps Malphas summons advanced (3x normal HD) abishai? He'd still need more than 1d2 with a 55% chance.
I would suggest this.
Summon Baatezu (Sp): Twice per day, Malphas can summon 2d6 advabced abishai (of any color) or a horned devil. The abishai are advanced to maximum hit dice. This ability is the equivilent of an 8th level spell. (Caster level 20th?)
along Shade's lines...
Summon Ravens (Sp): Twice per day, Malphas can summon 1d6 fiendish giant ravens or 2d6 fiendish raven swarms. This ability is the equivilent of a 5th level spell. (Caster level 20th)

Moving on from there, Abishai are all low low CR. Under 10, if I remember correctly. It wouldn't be out of line to let him summon 2d6 to 3d6 of them, if that is the only devil we are going to let him summon. I think all dukes should be at least capable of summoning something CR 13 or better, as pit fiends can summon a horned devil 2/day.
As a possibility, perhaps Malphas summons advanced (3x normal HD) abishai? He'd still need more than 1d2 with a 55% chance.
I would suggest this.
Summon Baatezu (Sp): Twice per day, Malphas can summon 2d6 advabced abishai (of any color) or a horned devil. The abishai are advanced to maximum hit dice. This ability is the equivilent of an 8th level spell. (Caster level 20th?)
along Shade's lines...
Summon Ravens (Sp): Twice per day, Malphas can summon 1d6 fiendish giant ravens or 2d6 fiendish raven swarms. This ability is the equivilent of a 5th level spell. (Caster level 20th)