This one would come down to GM discretion. The principle is that specific rules trump general rules. Both the Hellknight commander spell-like ability and Summon Good Monster feat are specific rules that override the general rule of the Summon Monster spell line, and the GM has to decide which of these two overrules the other. Given the context, I would say that the SLA takes precedence and you do not gain the ability to expand this SLA's list of monsters.
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The ability in question (Summon Devil) only allows you to summon a very limited list of monsters based on your level.
Since you don't have the full list of summons available to you I don't think an ability to expand that normal list would work.
In other words, the ability only lets you summon three specific creatures from the normal list and all others are banned for that ability. Even if you expand that original list, the fact that you can't summon anything from the list other than the ones specifically called out doesn't change.
Try thinking of the hellknight as a character who has desires, goals, ambitions, motivations, and a personality all his own. Such a hellknight gains the ability to summon devils because devils are cool, because they match his goals and ambitions, because he has so much in common with the devils' personalities, etc.
In short, he summons DEVILS because he wants to.
Which pretty much makes this guy evil. He's freely using a spell with the [evil] descriptor to summon Lawful Evil minions to do his bidding. If, somehow, he is NOT evil, then he wouldn't choose devils for this purpose - nice-guy summoners naturally choose other things for their summons.
A man who LIKES devils and LIKES summoning them and LIKES bending them to his will would not simply decide "And now what I really need is to summon despised yucky good monsters that I don't like, that don't like me, that don't understand me, and that won't want to do many of the things I want to make them do."
If your character is nothing but a bunch of numbers on a sheet of paper, and your goal is to make those numbers bigger (more powerful), so that you can "win" whatever you're winning, then by all means, find whatever trick works for you.
But if that character is supposed to be a person with a story and a personality, then what on earth is your reason for diluting his personality and cheapening his story?
And, if by chance, you actually DO have a wonderful personality and an excellent story for a devil summoning hellknight who has a great in-character and thematically story-appropriate reason for using Summon Good Monster, then any sane GM in the world would be right on board with having you use it.