I recently purchased one as well at 3.5 inches. Ive had him for about a month and a half and so far little to no growth rate.
What size tank do you have yours in?
Mine is currently housed in a 55 gallon which imo is perfect for the sanchezi seeing as though they only grow to be about 6-7 inches in length. Excellent filtration is required with plenty of plants and driftwood to provide cover. I think a power-head is definitely a good purchase to produce a current which mimics their natural habitat.
Water changes are key for growth and overall health of the fish. I do 2 water changes a week at 40% and use Seachem's Prime as a water conditioner. Great product.
The Sanchezi is definitely a solo species as are 95% of all serras. Do not add tank mates as they are seen being a potential food source.
The sanchezi (as well as every piranha) should have a varied diet. Some people on here have successfully gotten their piranhas to take pellets, but that can be difficult to do although definitely recommended for both health and color. I feed mine a variety of raw shrimp, tilapia, and night crawlers. Live food should only be offered as a "treat" because of potential parasite infection. If you are going to feed them live food make sure the feeder is quarantined first to make sure its okay for digestion.
It depends on the fish, but my sanchezi loves interaction, meaning he likes to finger chase.
Hope this helps a little!