[cs_gardener]

Shrimp are a real pain to try and catch them all. I moved mine from a 10 gallon tank to a 20 I was setting up in it's place. I trapped as many as I could by baiting. Then after I removed all the plants I netted as many as I could. Finally, after draining all the water to below the substrate level I caught the remaining shrimp (I could see them because they'd "flip" to try to get back into water) and they transferred to the new tank with no problems. I drained some water from a bucket into which I'd tossed some plant into my bathtub and discovered 2 adult cherry shrimp that had managed to escape all my efforts. I picked them up very carefully by hand and they're still going strong 2 months later (I put them in the nearest aquarium and that one had no other shrimp so I can see they're still doing well). I don't think you'll harm the fry if you try using a gravel vac, it would most likely be less stressful than what I put my shrimp through. I didn't have that option since my tank was too close to the floor to get a decent siphon.

I think I'd wait at least a month to see if the gravel changes the water hardness, but that's just my best guess.

Good luck with your tank! It sounds like an exciting project.