Dont mix water from another aquarium with your own. All you will adding is anything bad from their tank. Beneficial bacteria lives on surfaces. Substrate, glass, decorations etc, but mostly filter media. Very little beneficial bacteria is in the water, so no real gains and a lot of risk.
As to fish in / fishless cycle. We get a lot of traffic here when fishless cycles progress as they hoped. Fish in cycle is relatively safe if done properly. Pros and cons for each. One method will suit one person and the other method another.
Fish in. Pros
- Something to look at on day one.
Fish in. Cons
- Lots of water changes.
- Fish live in waste until cycle establishes.
- Everything is cycled before you add fish.
- Can be quicker to a fully stocked tank.
- Several weeks before you can add fish.
- Frustration when they don't progress how you expect.
Probably other benefits/issues i havent thought of. I never go telling people which route to take, but more than happy to advise on a process to cycle either method.