[MENTION=530181]LeSexyFishorse[/MENTION] - Well I am not sure i understand the way you expressed chain line. Chain line is the distance from the center of the bike to the center of the chain. In the front, if you have only to rings then it is the distance to the middle of tthe chain rings to the center of the bike. In the rear it should be from the center of the bike, hopefully the center between the drop outs, and the center of the freewheel/cassette. The distance should be the same, front and back.
Can you adjust the cassette/freewheel? Yes to a small degree. You can add or delete spacers on the axle to move it. The consequence is the need to dish the rim to center it. An alternative is to use a BB spacer if using a freewheel if it needs to move out, otherwise it is a removal of a washer on the axle. To move it the other way is to add a washer to the drive side.
How much clearance do you have when the chain is in transition from the smallest sprocket to the next sprocket on the rear? The clearance is dependent on the frame maker.
My Colnago came with a 6 speed freewheel. There was plenty of clearance.
P9161255 on Flickr

The freewheel was replaced with a 7 speed without any other adjustments.
PA121374 on Flickr