Retics are pretty slow growing iirc. I started with them ages ago. They are very intolerant of water parameter changes so constant monitoring of water parameters is needed. Once they get larger, they become a little bit easier though. The key is getting them to that point.
With regard to food, you'll find yourself feeding them blackworms and possibly small rosy reds at the onset. Then you can try to experiment with cut fish, shrimp, squid, etc. The problem is that their mouths are so small when they are young that you'll be cutting splinters of food for them. Frozen mysis shrimp and krill offers another option, but it will take some time to get them on it.
I've kept all of my rays in some sort of current going, along with a lot of oxygenation. Temperature of my tanks is about 78-82 on average. Hope this helps!