I'm not much of a leader line person, but recently I've been using fluoro leaders and I like it.

 

So I have a question. If I'm using 30lb braid, which is usually the diameter of 8lb mono, is it fine to use 12lb mono or fluoro leaders off of that? Will it work fine?

That's about perfect...for me. With 30lb braid I stay under a 15lb leader, makes my connection knots feel more secure and the diameter difference is very little. 

It'll work just fine.

Should be alright. Just double check the difference in diameter to see if it is close or not and choose the appropriate knot. Some joining knots are ideal when the line diameters aren't about the same. 

I like the fg knot and Alberto for line to line knots