They require the same things all dogs do, however, they are larger and smarter than most breeds, some can be stubborn. They are not by nature aggressive.
They are an instinctual guardian, so yes socialisation is a must, getting them used to children, other dogs, bikes, trucks etc is straight forward when they are young.
This needs to continue as they get older. Consistent exposure to new things helps them understand how to discriminate what is suspicious and what can be ignored.
An unsocialised dog thinks everything is suspicious because they see so little of the world from behind a fence.
As for being jealous, all dogs can be protective of resources, but the process is the same whether we are talking about guarding food, toys or you. You remind him all food, toys are yours you share these things and your time with him at your discretion and when people call round to visit or your family want to be close to you he must give them space. It isnt his place to decide how the house operates.
A well socialised dog, with a sensible owner who is of stable mind and character will not attack anyone, Rotts are known to be a wait and see dog, they are not reactive without cause. Only dogs that have been mistreated, badly trained or isolated are likely to attack anyone. This applies to a jack russell or spaniel as much as a Rott, the difference is that a Rott is 50kg+ with a head the size of a melon so the problem that exists with all dogs that have been badly raised is magnified with this breed.
Your dog will enjoy walks, but as Rotti416 says, build him up to it, his joints are still developing so take short walks initially.
Once he is an adult he can walk for hours at a time, but again get him used to it, bring water too as they need a drink on longer excursions.
The small yard isnt a problem, my backyard isnt big but there are places to walk her (and run her) nearby to drain her energy and get out in the world for different smells and to interact with more than just her family.
They can live outdoors but do better with you inside, you are their pack and they want to be with their family. Being alone outside isnt ideal for most social animals.
As to whether you made the right choice, if you are happy to put in any extra work these dogs can need now as a young dog, you will get it all back many times over when they are a settled, confident, reliable adult. You can really do almost anything with them if you spend the time building your relationship. Use firm but kind training methods, they dont respond well to rough or heavy handed methods. Kind training doesnt mean you let the dog walk over you, its not a strong vs weak thing, its all about attitude, you sent the rules on what you expect from him, he will understand if you keep to them consistently.
Adolescent dogs can be a handful, some will test you a lot of the time to see what they can get away with, but they start to settle from 1yr onwards and really start to calm down after 2yrs but it varies, some dogs can be easy from the beginning if you dont set them up to fail and use methods to drain their excess energy whilst training them to do new things and building trust and respect between you as you go.
If you have doubts about your choice, find a working dog club near you, a normal dog club might be fine and be worth talking to, but a working club will understand what this type of breed needs to help them mature into the family member, friend and guardian you want.
They arent for everyone, but you have him now in your care and this is an opportunity to have the best friend you ever had in your life. I would worry what might happen to him if you didnt keep him.