Hello; With already established tanks you can get a startng colony of beneficial bacteria (bb) which are one important factor in cycling a tank.
There are many threads on cycling already archived in the forum.
It may save time if you post briefly what you understand about cycling or ask a more specific questions.
Putting rosy reds in for two weeks and doing water changes is fish-in cycling. The live fish produce waste which has ammonia. The ammonia feeds the bb and they change it to a less toxic form. With already established tanks you can get bb from various surfaces such as plants, gravel, filter media and such. With the rosy red starter fish and some bb from the other tanks the bb colonies will grow to match the load of waste products from the rosy reds. If you add more fish slowly the bb population should be able to increase quickly enough to utilize the additional ammonia. If you add too many new fish quickly the bb population may take a while to increase and an ammonia spike can happen.
Good luck