Samus - Steel. No secondary typing to account for the beams because unlike Pokemon types, which are both offensive AND defensive, the beams would be strictly offensive in nature.
Mono-Steel though? I'd like Normal-steel, but Steel works. Give her a giant special movepool full of beams and such. Tri-Attack, Hyper Beam, Flamethrower/Fireblast, Icebeam/Blizzard, Energy Ball, Shadow Ball, Aura Sphere, Signal Beam. Basically Genesect's special movepool... only with Sheer Force, because then they all get rediculous.
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I'll just say that I think Peppy would be a Ground type with Forewarn while Slippy would be a Water type with Analytic (or Klutz? :p). I suppose they'd have a moveset similar to Fox and Falco, since I have nothing else to go off of.