It may or may not work...it just depends on if you're willing to dish out the money for the swordtails and then maybe watch them get killed. Cichlids are unpredictable, some of them are really calm and others...not so much. I've never had any shellies, but the ones I have observed in other tanks seem to stay near the bottom most of the time, while the swordtails will stay closer to the top/middle mostly. So...it just depends on if you feel like taking the risk. Hopefully when you put the two together you will have a separate tank ready to go for one group just in case.