Can green swordtails go in a the same tank as guppies?

They will be OK together provided you don't overcrowd them and overstock the tank. If you follow the basic rules of fishkeeping which are. :- 1 inch of fish needs at least 1 gallon of water. :- Every tank must have a permanently running filter. :- Every tank must have at least 50% of its water replaced every week. Stick to the above rules and your fish stand a chance of survival. Fail to look after the fishes water as above and I can guarantee that your fish will be constantly getting sick and will live a much shorter life than it should.