The Betta will definately devour them as bettas love their shrimp. Perhaps really mature shrimp won't get eaten, but one of the nice things about cherry shrimp is how quickly they multiply. I bought ten once and now have several hundred. I think you should leave them in your pleco tank for a few months, wait until you have eggs hatch and then once you've got baby shrimps, wait until you get a few adults and maybe throw two or them of them in with the betta. I have a betta but I have never tried it. I've seen him devour freeze-dried shrimps so no part of me wants to give it a shot. Someone else might respond with experience with that combination. But seriously, consider leaving them in the pleco tank, wrap a piece of women's hose around the filter intake as baby cherry shrimp are extremely small and WILL get sucked in. In a few weeks, maybe a little longer, you'll have more shrimp than you'll know what to do with and an extremely clean tank! I've read some people put a bunch of shrimp in a tank at night so that they clean up debris and algae and in the morning they make good breakfast for everyone else. I've never had a cherry shrimp survive any of my other tanks, except my guppy and puffer tank. But I never see baby shrimps in that tank. I bought them with the purpose of being bottom cleaners but I took the safe route and waited until they multiplied to try it. Because of that I rarely recommend them for community tanks unless they are full adults, and even then with caution (only ones with smaller type fish). So instead I have a shrimp tank with one chinese algae eater and enough java moss to take up the entire tank. It's actually one of my more entertaining tanks, especially because you can barely see through the java moss, not even at the surface. But seeing a tank just swarming with cherry shrimp...wow. Downside? Water changes are a *****. I always have casualties.
Hope this is somewhat helpful even though I didn't really answer your question.
