So my daughter and I are driving down a country road when all of a sudden she screams DAD, you almost ran over the turtle!!! I look at her then back on the road and say Honey, there is no turtle out here, you sure? She says yes. We do a you turn and then another to get to where she saw it and sure enough there is a turtle, on its back or flipped over on its shell, in the middle of the road. Who would have thought. Hell, I'm still trying to figure out how she saw it when driving at least 60 MPH. Anyway we get it, no idea what kind it is. Seem to recall it having a yellow stripe/line on its head or side of its neck.
Anyway we look around a bit and don't see water. I'm ignorant of turtles so just assume it needs water. We get in the car, she is holding it, and we come up on a water way, spill way, and believe that is where it walked from and probably heading to the other side in which cattle roam. So we set it over and it walks off.
Totally unrelated but since you mentioned turtles that is all I got

With regards to guppies and brackish water I've read a few threads with people successfully keeping them. Also mollies. Both are a good live food source. Mollies typically in the display. Guppies away from the more aggressive eaters so in the sump or refugium. Also those bumble bee gobies.
Should be possible. Oh - also Robby cool story on the turtle. I think that is super cool.