There are many fish that can go from fresh water to salt, (look at the trout that goes from saltwater to fresh to lay their eggs).
But I don't think trout can stay in saltwater long term. There are many fish that live in brackish areas where the water will be anywhere from totally saltwater to totally freshwater and everything in between. Some fish can handle this type of environment while others can't.
I also recall hearing about an eel that does the same thing as trout.
These guppies will be put in a seahorse tank (where there can't be too many fast moving fish or aggressive fish). They also will feed on small brine shimp, algae, copepods and well just about anything I put in for the seahorses. Also, being that guppies breed fast, the seahorses can feed on the guppy babys to which they do eat.
I kind of did the same thing with mollies years ago (I won't date myself too much by telling what year it was, LOL). But mollies can be a little more aggressive than guppies so I didn't actually let the adults mix with the seahorses.
What type of sea horses are you getting (or do you have already)? Many of the tank raised ones will eat frozen foods (like mysis shrimp) so live foods (like baby fish and brine shrimp) aren't as important as they were many, many years ago.
Guppies are the first fish I've had and bred. I've always liked fancy tail guppies and the way they looked (and look even better in marine lights) and now I have the best of both worlds.
Also one of the first fish I bred. I bet they do look good with actinic and full spectrum lighting.
Why would anyone want guppies in a saltater tank?
Because nobody else has them in a saltwater tank.
"Simply put, you believe that things or people make you unhappy, but this is not accurate. You make yourself unhappy." Wayne Dyer