All the common livebearers will acclimate to SG 1.005 just fine. All the traded varieties of mollies and all guppies will adapt to higher salinities as well, right up to full strength seawater. With mollies this is easy and only takes 60 minutes, tops, to go from freshwater to marine conditions. Guppies are not so easy, and it takes several weeks to adapt them safely. You can also buy "marine guppies" used variously as lab animals, mini-reef denizens, and feeder fish for marine predators.
Pike livebearers are normally found in brackish water and less commonly in freshwater or the sea. In other words, the precise salinity doesn't matter much to them. The same holds true for swamp guppies (Micropoecilia picta).
From the other poecilid livebearers, anything that does well in hard/alkaline water can be expected to thrive at SG 1.003, perhaps even 1.005. It might not be ideal for them, but it'll be unlikely to kill them either. Mosquitofish for example do fine in brackish water. The same probably holds true for the Goodeidae, though relatively few are normally found in brackish water. Xenotoca eiseni and Ameca splendens are both salt-tolerant and used in labs and mosquito control thanks to this. You can reliably expect them to adapt to SG 1.003-1.005 without problems.
Among the halfbeaks, most are soft/acid water fish, though Dermogenys pusilla and Nomorhamphus ebrardtii are found in brackish water as well. I've seen Celebes halfbeaks (N. liemi) kept at SG 1.003 without problems, but it definitely isn't what they want. On the other hand, Zenarchopterus halfbeaks are usually brackish water specialists, and will not do as well in freshwater tanks.
If you want more info on brackish water livebearers, you could look over my
Brackish FAQ, or consult either my brackish water fishes book or the Aqualog one, both of which have detailed sections on livebearers and halfbeaks.
Cheers, Neale
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