You don't have to use bizzare keys like C# , it will only make it harder for you to play.

usualy , famous keys of that are usualy used for guitar are E,A,D & G , cuz you can use open strings in the song , which usualy helps.

You can put minor chords in a major scale progression, like Amin chord in a Cmaj scale.

Each song has got to have a Key, may be some different Keys in some riff inside, but again each riff must have a key.