Assassin snails wouldn't actively hunt shrimps, the healthy shrimps are much faster and can easily escape the snails anyways... only if a shrimp is weak or dying (or already dead), then assassin snails will help clean up the carcass.
Whether the shrimps will crossbreed depends on their type and genus, you can refer to this chart to find out: http://www.planetinverts.com/Will%20...nterbreed.html (you can also just do a google search to find many other charts too)
If you want to minimize stress and maximize breeding/shrimplet survival rate, then its definitely best not to have any fishes in the tank... though fishes like otocinclus are confirmed safe with shrimps and shrimplets, so they are often kept in shrimp tanks.
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