On the prerequisites for Alternate Ranks the Rank requirement is that the firt Rank it can take the place of or is it the Rank afterwards.

For example I want to take the Moritant Reaper Alternate rank with a Rank requirement of 6. does that mean as soon as I spend over 5999xp and qualify for Rank 6 that I can take the Reaper alternate and replace rank 6 or do i have to wait till ive spent over 7999xp and will be going to rank 7 and replace Rank 7?

We played it that it "replaces" the listed rank. So, in this case, it replaced the Death Adept branch of the tree.

You must spend 6000Xp then you take this alternate.