Okay.....4-5g....IMO-I wouldn't add any fish...but maybe 3 dwarf cory cats...but that could be risky in 4-5g and they really do better in groups of 6 or more
Maybe 3 otto, once the tank has matured some in about 4-5 months-they would work
snails, different shrimp like red cherry or the yellows
Not a lot of swimming room and with limited swimming room come stress and more chances of the Betta catching them even with hiding places and plants...check into kuli loaches-I know they need to be in groups, I have never kept them but they are small and burrow I think....
He should...however, if he is working on the one in the back that is harder for you to see, he may be doing this for a reason...some fish don't like to be watched and can feel safer and do a better job, so you may want to leave it to help increase you chance for success...now that I think about it......
Fish will often eat their eggs/fry when they feel threatened-as odd as this sounds they would rather get the nutrition than let something else get the nutrition-they do this so that they will have more energy to spawn again in a safer place to improve chance of success.
So if I remove the one that's farther away (that he's built the most in) and leave the one that's close to the glass he'll continue to build in that one? Thank you!
Oh and... one more question, if you don't mind-
there are two cups in his enclosure. He's working on the side in one and he's working on the middle in the other. Should I keep them both in there or remove one?