AS you all know I do love a challenge,I've got a friend in the USA that has set up a small test tank and has successfully acclimatised guppys to marine .
Why I don't know but he's done it ,
Weirdo
Dave

Back in the day I used Guppies and Mollies as GHA eaters. Mollies are better at it, and grow massive 6"+ in marine tank

I do have a spare 8G (18*10*10) tank that I am thinking about Guppies and Goniopora, or Guppies and Mangrove