can i still play SP if i also play MP?

no, you cant mess up your singleplayer while playing multiplayer.
You have to decide via launcher what you want to pick Start Singleplayer or Start Multiplayer
Both at the same time is not possible.
Also the datas regarding your characters in Multiplayer are going to be saved on the server and nowhere on your computer.
The only datas which are used from both options are the client side datas, such as systems, ship-information (but NOT what is mounted on your ship) and of course the settings for your game (keybindings, and such)

You can safely join the server for multiplayer and still continue on your singleplayer campaign later on.

*moved*