. I'm in the process of learning my scales...I'm working on C and G first. I know I can use mostly I-IV-V chords to play a song, but can I just "add" other chords as I seee fit, in order to liven up my song?
Yes, and no. If you are in the key of C and you want to go to G, there are a lot of ways to get there, i recommend you do a 2-5-1 in the key of G. In which case the baseline would be A,D,G.
Is a bassline the scale? i.e., if I'm working in the key of C, is my bassline just C,D,E,F,G,A,B,C?
Again, Yes and No. This question and the other question for that matter, is best answered by checking out the Lessons area under the lessons tab. Just click on base lines.