At about 3 inches mine went through a stage of not growing for a couple of months, no discernible reason in terms of stress, health, appetite, etc. So I started feeding them about 30% freeze dried mysis shrimp in their diet, which got them growing again. Just my experience.
I'm in favor of using UV for sensitive fish, have a good bit of experience with them. I'd call them optional, not a necessity, but if you do some research they have benefits beyond pathogen control, including a positive effect on fish immune system and redox (an element of water quality). The theoretical benefit to water quality is something I've seen in the real world. IME running UV makes your existing filtration more efficient. Put another way, with UV it takes less filtration to get excellent water quality. Not saying you should skimp on filtration, but with sensible filtration you can get the same water quality as some who way overfilter.