I haven't kept swordtails in a long time- I think if you wanted to get one gourami, two swordtails and the three cories that would be okay in a ten g. However, While swordtails aren't as prolific as say guppies, they are still livebearers. Which of course means that even with the gourami acting as population control, you might end up with more swords that your tank can handle. The nice thing about egglayers like tetras and rasboras is that breeding them successfully is more of an effort. My harlequins lay eggs all of the time, but I have yet to see any offspring. You could get two males, but they might stress each other out- two females could arrive pregnant and (just in my opinion) the tetras would add more to your display in terms of activity, etc. but it's all about what appeals to you.