Peacock:
Copy Robot/Mega Man? from Mega Man Powered Up* (Her Avery System is kind of sort of (But not really) like a Variable Weapons System), or
Bomb-Man from Mega Man Powered Up (Since he shares her traits of immaturity and bomb-chucking; two different sizes, even—George and Lenny, anyone???)
Parasoul:
Napalm-Man from Mega Man 5 (For the obvious "Napalm" link, and Napalm is kind of blatantly also a soldier)
Painwheel:
Metal-Man from Mega Man 2 (Because in addition to sharing large metal cutters, Metal is weak to his own weapon, which is kind of similar to Painwheel's self-mutilating moveset), or
Gyro-Man from Mega Man 5 (Since they both can fly via the installment on their back)
Filia:
Proto Man from Mega Man 3 onward (Honestly the links are weak here, but Filia was the first Skullgirls character (Development-wise) and Proto was the first Robot built (Canonically); canonically
Proto's hair is brown, but Samson could also be yellow in representation of his trademark scarf), or
Search-Man from Mega Man 8 (Even though Search-Man's two heads/Filia and Samson share an objective, they squabble often; not really sure what color Samson would be here)
Cerebella:
Clown-Man from Mega Man 8 (They're both oddly-armed circus performers, seems kinda natural)
Ms. Fortune:
Slash-Man from Mega Man 7 (Since he kind of has catlike agility as well as claws)
Valentine:
Shadow-Man from Mega Man 3 (Because...both of them are blatantly supposed to be ninjas?)
Double:
Yellow Devil from Mega Man...
or any other Devil, really (Since they share the characteristic of being amorphous and cyclopic)
___
Squigly:
Skull-Man from Mega Man 4 (There are a shortage of Robot Masters in this department to begin, but giving Skull to Squigly over, say, Marie seems logical because Marie already has white very prominent in her color scheme; Leviathan and/or his flames might take on a cyan color, like the color of the
copy-weapon version of Skull-Man's weapon)
Big Band:
Guts-Man from Mega Man (Not only does he have "big and bulky" going for him, but
Guts also canonically enjoys karaoke, linking him (Albeit loosely) to music)
Beowulf:
Concrete-Man from Mega Man 9 (His palette might not play nice, sure, but Concrete is really the only "strong-man" Robot Master that actively incorporates melee into his moveset)
Eliza:
Pharaoh-Man from Mega Man 4 (For reasons I'm assuming are obvious)
Robo-Fortune:
Grenade-Man from Mega Man 8 (Apparently Robo-Fortune has self-destructing as a gimmick, and this explosive, rambunctious guy—the only other Robot Master weak to his own weapon—seems a perfect fit)
I did have some more planned—Isaac, Aeon, Venus, Adam, Panzerfaust, Annie—but I figured I was already going overboard and should stop while I was ahead. Also I didn't have any ideas for 2/3 of the characters that are allegedly needed for Chapter 1 of the canon storyline (Umbrella and Black Dahlia), and I figured it would be wrong for me to jump ahead of them.
* Sorry about the poor image quality, but there aren't many pictures of Mega Man?/Copy Robot floating around to begin with.