Red Zebras (
Metriaclima esterae) are mid level aggression by mbuna standards, and by no means 'calm'. I can only urge you not to keep them with frontosa. Electric Yellows (
Labidochromis caeruleus) are milder in temperament. Personally I would keep no mbuna with Frontosa, but I'd give them a higher chance of working out than any other species. Electric Yellows should not be kept with Red Zebras, since cross breeding can occur.
I am pretty clueless when it comes to Victorians like Zebra Obliquidens (
Astatotilapia latifasciata), and I have never kept
Nimbochromis venustus, but frontosa should not be kept in pairs. Your initial idea of 4-5 in a 125G was much better. As others have pointed out, if you get them as juveniles, they can get used to tolerating other Tangs in the same tank. Altolamprologus are probably the most popular choice, but many lamprologine can work. Yet, having tank mates will influence the fronts breeding behavior, and maybe stop them from breeding completely. You should also be prepared to pull out any tank mates at short notice, and you should have other tanks available in case that becomes necessary. I have a hard time imagining adult fronts not eating cyps and other smaller fish in a 6' tank - even though they probably get on fine while the fronts are smaller.
I am keeping 7 fronts (1m,6f) in a 8' 240G tank with many other Tanganyikans. It's a great tank to watch, but I am always keeping my eyes on it in case one day the fronts get hungry. Also, I've got only 3 fry in 5 years. Without tank mates there would be hundreds, but the other fish eat the eggs before the frontosa females can get them in their mouths.
By the end of the day it's your choice how much of a risk you want to take. That can also depend on what plan B you have in mind in case something goes wrong; ie is this your only tank, or do you have other setups that you could use to evacuate tank mates to in case they no longer work out with the fronts. Another choice you have to make is tank mates or optimal breeding conditions for the fronts. You can't have both.