If someone chooses Singapore Airlines, they will be based at Singapore Changi. My only rule for the world states that your airline needs to be based at an airport in the country where the 'real' airline operates from, i.e Malaysia Airlines out of an airport in Malaysia, South African Airways out of South Africa, etc.
Its your choice if you would like to have your main hub, the same as the real airline or whether you would like to have your main hub at another airport within the country. I feel that there is no need to restrict the hubs, aircraft being used or creating routes. That is solely up to you.
The only rule is, you have to play as a real airline, based out of your 'home' country and you will be free to choose what aircraft you will deploy and which routes you will create.
So yes, to answer your question, the player who chooses Singapore Airlines, will have their main hub at Singapore Changi. This player can freely choose which other hubs they would like to open worldwide in accordance with the hub limits.
