Some of the monsters I gave actually can't have a location ascribed to them yet because I didn't set up some cards for them. For that, I'll need to come up with a few more location cards, so I'll get to them in a second. For the ones that can be located...

-Goomba, Koopa, Paratroopa can all fit in Mario locations like Princess Peach's Castle, as you mentioned.

-Slashers can be placed on either the Ishimura or Titan Station.

-Cerberus Troopers can be placed on the Citadel.

-Reaper Husks can be placed on the Collector Base.

For the rest, I should probably throw a few other ones in. So here, have some more locations:

Tuchanka - VG -
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-Thresher Maws can be placed on Tuchanka.

Denerim - VG - City
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The Deep Roads - VG - Cave
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You can place Hurlocks and Ogres on either Denerim or The Deep Roads.

Green Hill Zone - VG - Hill
Image: http://supersmashbroschronicles.webs.com/poi.bmp

You can place Motobugs in the Green Hill Zone.

And ah, what the heck? I'll throw in some more Mario locations for you to go wild with.

World 1-1 - VG - Plain
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Rainbow Cruise - VG - Spaceship
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For ease, Goombas and Koopa Troopas can go in Princess Peach's Castle or World 1-1. Paratroopas can go onto any of these places.

Hope that helped!