Here is a picture of my Betta, not very interesting in terms of colour but easy to see in the tank as it is quite dark! He is with some high grade red cherry shrimp and it is fairly densely planted (sounds better than the truth - overgrown) but I am sure he wwill be eating some of the baby shrimp when he sees them. If you get to the stage of having a cull tank for poor grade shrimp then putting a betta in that would be ideal, otherwise it is a risk keeping shrimp with a betta so only try it with your least favourite shrimps in a well planted tank. Betta are fairly easy to keep (as other tropical fish) and interesting as they will interact with people who go near the tank and at feeding and tank maintenance time.
For creating carpets of moss, I used to use pads/squares of stainless steel mesh, on which I put the moss then wound fishing line to hold the moss in place. For attaching plants/moss to wood or rock though the superglue is much easier, it may need to be a certain type/make though??? I always buy it from my shrimp supplies supplier.
I saw my friend earlier and the apistos seem to be doing well so most likely I will try those in the 35L aquarium, so won't have many. Not got any further than that at this point. I wouldn't even consider putting shrimp in with apistos.
What you have been reading about 'ageing' water I suspect is to get chlorine from the (tap) water?
The friend I saw today (with the apistos) bought a 360L aquarium about 10 years ago so we had to modify/strengthen his sideboard to take the weight. His floor is concrete so that wasn't an issue. Even with just the 3 tanks before I moved the wiring was crazy with each tanh having, heater, light, filter - looked a real mess, but it didn't actually use much extra power as only the heater(s) were >10W.