Growth of Frontosa

first i would say they are in a rather small tank, they need a six foot long tank to escape aggression, a 6' x18" - 100 gallon is enough, of course larger is better. For food the NLS sinking 3mm pellets are what lots of people feed fronts, right now i have some thera+ and regular spectrum food. I also feed sinking carnivore pellets too, mine seem to like all the above. IF you decide to change foods fronts take a long time to warm up to new food, like a month or so, i try to introduce new food gradually so they get used to it over time, feeding little of new food, and cut back on the regular food they are used to.
I sounds like your fronts are maturing and are needing space mostly.

herb

you are having aggression problem cause your frontosa is getting to mature age. With a 3' long tank, it is very hard for the passive fish to survive. They will not die from beating but usually prompt to disease deal to the stress level.

What you feeding is fine. I feed mine with omega1 and mostly self prepared fresh food. Actually, I was quite surprise yours are 1.5 year old and at 8". I don't recall much people posting with that kind of success with growth rate. If that is true, 18 months in a 36" long tank and you get to 8" 7" fish, I wouldn't change their diet.

The smaller fish could be female. And it also could be less aggressive male.

18 months in a 36-inch tank... consider yourself lucky to not have lost fish already. There is a high probability that the aggresion to get worse. A major sign of stress is the others hiding all the time and when they stop eating, death is not too far away. Their ammune system will weaken from the stress and they will get some sort of illness (like Charles said). I strongly suggest you get a tank with at least a 6' foot print (ASAP).

Do you have a hospital tank? Do you have meds on hand?

Not that you'll need it, I just wanted to point out a couple good meds that I keep on hand (Kanamycin & Metronidazole... from JEHMCO). Frontosa are expensive fish and I recommend people have some good stuff in their medicine cabinet BEFORE they need them.

Good luck,
Russ