1) That's what a line input is for - so you can take the line out of something like a pre-amp and connect it to the line input of something like a converter. If something has a "Mic/Line" input that means it can accept either - mic or line - the rest is just a question of gain.
2) There is no right answer. If you can get it "right" when you're recording it makes it easier to mix. I hate plugins... but that's a personal opinion. If you feel they can do the job for you, use them. I'm also of the opinion that you should "always be mixing" when you're recording... making the necessary sonic and musical arrangement decisions as you go along rather than waiting for someone else to make those decisions later. As I'm sure the moderators on this board will agree in principle, the fact of the matter is that I'm quite the fossil and bloody few do it that way anymore [as evidenced by so many sessions being delivered to mix with 60-70-80-90+ tracks of information to be weeded out by "the mixer"].
Hope this is of some assistance.
Peace