I have never done Jewels with Dithers, but I have done them w/ Rift Lake species and SA/CA species and they do MUCH better w/ SA/CA species than they do with Africans. Try just getting ONE Jewel instead of a pair and mixing it w/ other mildly or semi-aggressive SA/CAs - I have kept them frequently w/ Jack Dempseys, Green Terrors and Convicts, but I have also kept them w/ Flowerhorns, Red Devils, Firemouths, Jaguars, plus Polypterus, and Corydoras, Hoplo, Pleco, Raphael, Sun/Eclipse, Colombian Shark, Pimelodus, Asian Bumblebee, and Synodontis catfish, and also Chinese Algae Eaters
Ottocinctus will get destroyed, single or by a pair. Go armored catfish or at least semi-aggressive, but keep the size under 8-10" or make sure the adult catfish have small mouths
Sounds possible. Jewels(depending on species and there are many) do vary in temperament but most should tolerate the tank mates mentioned. I have a spawning group in a 120g but they are very tolerant and allow other cichlids within 4-5in of their nest. Granted, no one enters their cave...
how many females does your male Jewel spawn with and do they tolerate/raise each others' fry?