t0m5k1

Not all domain providers will allow you to change the nameservers of a domain they sold to you as they want to sell you the rest of what you need for extra $$$

You can only have 2 name servers on a domain and it is not advisable to make them point to different DNS providers as they will both need to be authoritative and by having 2 different providers will mean you get 2 different SOA which will break fundamental DNS.

to change the nameservers will either be simple or hard, depending on the domain name provider it might take 24 hours for them to change the name servers or they may allow you to change them via web UI which could be just a 2 hour wait.

I purchase from the cheapest and use he.net for my nameservers.