The lack of reading does not mean anything is wrong, it just means there are so few fish in the tank that it's not showing up. Keep monitoring the levels... if the ammonia or nitrite goes up, do an immediate 30-50% water change. If you keep the ammonia level below 0.25ppm at all times (same deal with nitrite) hardy fish like angels should survive the cycle without permanent damage. They may survive levels up to 1ppm, but if they are exposed to this much ammonia for more than a few hours, expect permanent gill damage which will shorten their lives.
It might take up to a few days for ammonia to register with two juvenile angelfish in a 40 gal, but don't get more fish.
I wouldn't recommend electric yellows with angels... in fact there arent' very many fish that can be kept with them. Also steer clear of small fish like most tetras, as angelfish will snack on them when they grow a little bigger.