Welcome in snowy Maastricht

To answer your first question, it doesn’t matter by which mobile company you will buy your prepaid. For example, it doesn't matter if you will have KPN (Dutch mobile company) and will call to a phone number of someone who has Vodaphone or any other server. In the ‘Grote Staat’ in the city centre there are a few mobile shops who will help you for sure.
For second bikes, perhaps you can ask by the train station, next to it there is a bike shop. And if they don't sell bikes, they know for sure where to buy them!
Good luck and if there's anyting let me know!