Welcome to Tonmo! silicone, as KIA stated, will not completely adhere to plexi...but it can serve as a caulking... Epoxy might be a better bonding medium, but how about using glass partitions?
Looks like a fun tank idea...look forward to your progress!
They will eat the micro algae -> this way when it grows I can take out the over growth and feed it to the snails a treat.
I did not know that silicon would not stick to plexi-glass. That leaves me with two options. I do not want to see into the back two tanks. That would also cause confusion for any ceph that could spot food and not be able to get it. So I would either need non-transparent glass or just make the whole tank acrylic.
I'm positive you could use plexi and silicone. Since the water is almost the same height on both sides the plexi won't see any significant pressure from the water. As long as you silicone all 3 edges of the plexi it will hold quite well. I used plexi dividers in my sump and aquarium silicone, it took tons of effort to remove them when the time came.
Why do you want to set it up this way? It seems like those back triangles could be a real pain to clean, and if something dies in there a hassle to get out.