I appreciate your curiosity in terms of wondering what crosses will work. A lot of times, you could save yourself some time, space and money and just Google the net for guppy - swordtail crosses. Even a quick look at Google scholar showed no cases of a guppy-swordtail cross.

There are some studies which come up with that search but they point out some of the chemicals those fish have in common. Likewise they face similar and increasing threats from diseases (and that must sound familiar to guppylog members who have monitored questions here for a while).

It is certainly possible that I missed something. It is also possible that an aquarist claimed a cross which was simply a Xiphophorus x Xiphophorus cross and misinterpreted it. Just because something is written on the Internet (like there was no Holocaust!) doesn't make it true.

Then if you wanted to pair a guppy male with a virgin swordtail female (or females) or a male swordtail with a virgin guppy female of female in a controlled situation, then go for it. Controls would include basically using a covered tank or tanks, containing no other fishes and some distance from other aquariums. Collaboration with someone else gives you a witness. Video or photography would be useful too.

Certainly guppy males will pursue female swordtails. They will also even pursue female (and very startled male) egglayers too.

However there is the issue of the shape of the gonopodiums of each species. They are designed to fit the female genital opening of their species. The male and female organs are not designed to fit other livebearers and that is one of those classic isolating factors which are sometimes used to define species.

Here is a close-up of a guppy gonopodium:
http://www.zoo.utoronto.ca/roddlab/YunGonopodium.jpg

These images of Endler's and guppy gonopodiums from a German magazine show the hook so characteristic of guppies. The Endler's (P. wingii?) gonopodium has a hook, though it is not so pronounced.

http://www.diewasserwelt.de/endlersguppy3.htm

That other family of Mexican livebearers, Goodieds, and half beaks, Anableps (4-eyes), some sharks, California surf perch, even a genus of tetra (characin) and other livebearering fishes will have quite different male organs which allow for internal fertilization of ova.

Iwasaki, in his book on guppy genetics, noticed a couple of decades ago, that hobby guppies from some countries had gonopodiums which were becoming a little different from wild guppies or those of guppies from other societies. Whether that genetic drift could prevent some hobby guppy from being able to breed with others, some decades down the road remains to be seen.

Dawes book has a microphoto of a Xiphophorus helleri (swordtail) gonopodium on Page 56. It has an extra claw or hook, missing in the guppy gonopodium. It lacks that lower section present in the guppy gonopodium.

I skimmed through a few hundred images called up by various Google searches. I would be surprised to find a close up of a swordtail gonopodium on the Net. I didn't find one though.

This web page suggests that even environmental conditions may influence the size of gonopodiums. And female livebearers will recognize certain males and distinguish between them. There are a number of studies of that sort. I know this begins to sound like some of that unsavory spam which clutters e-mail boxes. I'm not trying to be crude, but reproductive organs are an" isolating mechanism", keeping some species of livebearers from mating with individuals of other species. A quick read suggests that there is a lot of research yet to do.

http://www.biology.wustl.edu/~lososlab/langerhans/gonop.html

By the way, 26 Xiphophorus species were listed in a 2004 article describing two new species! The beat goes on!

http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=all&search_value=Xiphophorus&se arch_kingdom=every&search_span=exactly_for&categories=All&source=html&search_credRat ing=All

One last set of concerns: I don't think that you planned on selling some of your crossed fish and defrauding people. But if you sold your guppy x Endler's offspring as just guppies or Endler's and forget to mention that they were a cross, that is fraud. If you sold them as an guppy x Endler's cross you at least honestly represented what you were selling and the buyer can plan accordingly.

Hobby swordtails, variatus and platys are something of a mish-mash of crosses between those species. Commercial mollies really can;t be called a species because they are hybrids, often of three or even four species. So crossing has been done and accepted. Well informed hobbyists know what has happened. And this fish do breed.

The ethical issues also surround some crosses which produce mules (a sterile cross of a horse and a donkey). There are crosses which look wonderful but will not produce viable viable fry. Since most aquarists aren't at all interested in breeding all of their fish, that is probably a non-issue with them.

A further danger has to do with the possible release of fish like the Flowerhorn cichlids. By most peoples' standards they are beautiful fish (sometimes selling for thousands of dollars). But if these artificial creatures are released in the wild they may spell the extinction of other fishes. If released in parts of Mexico, they could even bring about the extinction of parental species.

And our common platys have played a role in the extinction (in the wild of the beautiful Ameca splendens. Common Gambusia and bait releases are responsible for other extinctions.

The record of a FEW aquarists and commercial fish farms and even government agencies has contributed to a lot of bleak situations and ecological disasters coming from releases of fish in the wild. Those things have some people even lobbying for the ban of or severe control of aquarium keeping. If we start tossing around hybrids of two species, that will only complicate things further. (In some places there is talk of licensing a person for EACH species of fish they keep.) In Illinois the council for the states F&W department would like to allow NO ONE the right to keep Illinois native fishes, I'm guessing because of the damage that dumping species in waters which they are no native to. There has been talk of making UK fish shops have to get a license for each species they would sell. I don't know if that will happen, but those are the sorts of things which may happen if we, as aquarists, play fast and loose with our stock.