Uaru growth rates will vary depending on the fish. this will also vary depending on tank conditions and diet.
I have a 8 inch female that was less than an inch long 16 months ago. so that averages out to be around .4 inch per month.
I also have another unsexed uaru that is 7 inches that i got at the same size 11 months ago. so that averages out to be .5 inch per month.
I would say they should be around 6 inches at least within a year, and then slow down after that. they start to fill out vertically at this point.
I also just picked up two more uaru that are at least 8inches, and my guess is they are also 1.5 to 2 years old.
For a diet, mine get Hikari Excel pellets, Frozen blood worm, and romane lettuce. Its not a very varied diet, but its what they love and covers all of the essentials.
You will find that a lone uaru will be skittish and not eat alot. Unless you have a very busy tank, with other simular natured fish that he can use as an adoptive family. If you uaru was much larger, you wouldnt need to worry about other uaru needing to be in the tank, but it can be a crutial point to keeping uaru.