I have a trisome, 2 girls and a boy, cherry shrimps, a puffer, 7 highfin barbs and a bunch of boraras sp... 🙂 All except the boraras i suspect to be shrimp hunters.. 🙂
It's mainly the male pumila who's kinda behaiving territorial and beeing the rascal, occasionaly i see him haras a shrimp of it's own size but this realy is occasionaly and ussualy the shrimps are pretty fast and always escape.
I kinda antisipated to the possibility of baby shrimps beeing hunted and eaten and or maybe even some addult shrimps getting killed, so i made the setup like you did, with a very large piece of wood and made sure there is a rather large cave labyrinth bellow it. Constructed this with a few balls of panties filled with akadama bellow the wood as support and pilled up the substrate around it. Anyway it's going on for many months now and the theory seems to work, i threw in 30 cherry shrimp and they seem to breed fast enough to keep the population alive.. I still see addult shrimp also pregnant girls and baby shrimps comming.. I have no idea how many shrimps live in my cave labyrinth but they keep comming out. Also have 15 amano shrimps and i only see them rarely, they also are very fanatic miners and rather stay under the wood, i even see them shifting substrate around at the cave entrances. When i throw some cucumber into the tank it gets dragged into the caves to be never seen again..
I also see the pumilas d(r)ive in the cave, can't see what's going on, but i guess for hunting shrimplet burger.
For me the circle seems to be round and a goal achieved, do not have a massive amount of cherries but they breed fast enough to survive with a healthy population. 🙂