If I were you I would keep the guppies and cories and the snails. From everything I've read your not supposed to keep any type of pleco in a tank under 20 gallons..30 gallons is perferred because they all produce so much waste.
The molly well I've seen ones that were 4 inches long so I don't think in the long run you'd be able to keep it in your 10 gallon. The swordtail same thing they get just as big sometimes getting 5 inches in length. It'll take a couple of years for them to reach this size though so if your planning on upgrading to a bigger tank in the future you might be able to hang onto them for awhile longer.
I'm not sure about the killifish but from what I've read and heard from other members they get quite aggressive.
So here's how it would look if it were my 10 gallon
4 guppies
2 cories (would add 1 more as they like to be in groups of 3 or more)
2 mystery snails
and if your planning on getting a bigger tank soon you could maybe keep the molly and swordtail but I'm not really sure they like to be on their own. I'd think they would be happiest with 1 male to 2 females but you can't fit more in your tank.
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