UV Sterilizers do not help help or harm healthy discus. Your aquarium should be maintained well to ensure high water quality and the best way is through large frequent water changes. Domestic Discus do not require soft acid water. They will do fine at a pH 7.4 and 400 ppm TDS. The domestic discus is very different from a wild one and they have become used to very different water after over 50 years of TR breeding.
The most practical way to handle grow out of juvenile discus is in their own bare bottom tank. Nothing grows young discus out bigger and faster than a quality beef heart blend as the staple food supplemented with frozen blood worms, live black worms, Tetra Color Bits or similar granulated discus food and quality flakes. The flakes you are using are fine. No one food can provide all the necessary nutrients and it is best to train your discus to accept a variety of foods. I usually limit the beef heart to about 50% of their diet.
Beef heart Blends are messy foods and young discus benefit from very frequent feedings of up to six times a day. There should be ten gallons of water allowed for each fish. I tend to change 75% of their water two or three times a week.
The time to try keeping your Discus with other fish is after they have reached adult size which should be over five inches in 12 months. They will be larger if you follow my suggested guidelines closely. I don't recommend keeping discus with very many types of large fish. They are not an assertive species. They are best kept as the dominant fish in their tank.
I have kept six adult six inch wild Royal Blue Discus with Geophagus cf. surinamensis of about six inches, also a large Black Ghost and some Arowanas less than ten inches but these last two were only passing through. I did this when I owned a fish store and the aquarium was a six foot 125 gal.
The Geophagus were added when they were only about three inches and were eventually sold after about two years with the discus. Had they remained they may have become a problem but they coexisted without problems while I had them.
I am not endorsing trying to keep discus with large fish. I think the domestic discus are less suited to living with anything large than the wild Royal Blues I had. They were very well acclimated fish that went on to become breeding stock. If you really want the best for your discus you will keep few other fish with them and those you do should be peaceful. Fancy small Plecos, Hypancistrus sp and Peckoltia sp are good tankmates. They like warm water and also the same foods as discus. Some Tetras that do well with discus are Rummy Nose, Bleeding Heart, and sometimes Cardinals. Personally, I notice those who try to keep Cardinals are always having to buy more. They seem "burn out" at 85F. If you don't mind it try them. Blue Rams and discus do well together although Blue Rams are naturally short lived, most rarely grow older than 2-1/2 years but they are another good warm water fish.