Which of the common livebearers, guppies, mollies, platies, or swordtails, are the hardiest? Can they coexist in a 30 gallon aquarium tank?
First, I need to emphasize that there are different wild species of each of these groups and hybridization among the livebearers is rampant. Thus, any answer to your question would be a sweeping generalization at best.
That said, I would probably choose platies as the hardiest of the bunch with swordtails coming in a close second. Modern fancy guppy strains seem to be particularly fragile. Mollies can be very hardy when their requirements are met, but they seem to be less adaptable than platies and swordtails when it comes to water parameters. All of these livebearers prefer hard, basic water, but these conditions are pretty much non-negotiable for mollies. In fact, mollies tend to do best when kept in brackish conditions in aquariums.
Combining all four types together in the same 30 gallon aquarium may create some problems. For example, mollies can be major bullies and could make life miserable for the smaller, more peaceful platies and guppies. Male swordtails can be pretty obnoxious in this regard too. Not all livebearers are on the same page when it comes to water temperature either. Platies, for instance, prefer a temperature somewhere in the range of the low- to mid-70s Fahrenheit, while mollies like it warmer, between the upper- 70s and low-80s. The bottom line is, it won’t be easy to provide conditions that make everybody happy.