Get the biggest shrimp of the type you want, you can find!
Baby shrimp are smaller than a pin head.
If push comes to shove, the hardward store sells *egg crates*, in the light section (light disfuser), it's a plastic square mesh material, whos squares are big enough for shrimp to walk though, but small enough to keep fish out!

Like a shrimp safe house if you will.
Just suggestions. I've not done the above before. I currently have a 20g with mollies/cory's and shrimp. The shrimp are doing well enough that I find 10-20 every couple of weeks to move to my shrimp only tank. The shrimp were there first though, and i've a soft ball size or bigger clump of java moss.
Good Luck! Shrimp are awesome!! I like'em so much, I made a shrimp only tank!!