I can't answer your first question with any certainty, but I think you can mix and match.  I think that lines are meant to be used together because they're formulated the same way to give more predictable results. So, you probably can and I'm sure many people do, but don't quote me.

As for toning suggestions, do a strand test first.  I would also use a porosity equalizer first on hair that light to ensure that the whole head will process the same. 

When hair is that light, it can be extremely porous in places and not as much in others, which means that the color processes more or less quickly in different patches.  This could make some of your hair grab the base color (purple) while others turned out fine.

So, porosity equalizer, then strand test a few strands.  I would recommend standing in front of a mirror and using a stopwatch.  Keep checking the strand for the desired tone and see what time it took to process.  Then you can go back to do the whole head and know what time to process.  It can be as little as seconds to as long as 30 minutes depending on what product you use.  Read the label to see what is recommended for the product you pick, including whether the hair is to be wet, dry or shampooed & towel-dried.