I'm working on a different time frame than yours (the 17th century), but I am also looking for REA families in Roxburghshire. According to the General Register for Scotland, there were was a Patrick Rea born in Yetholm Parish to Thomas Rea on November 25, 1694. Also in Yetholm Parish was William Rae, born to Andrew Rae on March 1, 1691. There was a Dand (David?) Rea born in Yetholm Parish to James Rea on October 22, 1693, as well.
Meanwhile, in Ashkirk Parish, there was a Thomas Rea who had a daughter named Heilleine, born July 18, 1647 and there was a Johne Rea with a daughter named Margrit, born Aug 11, 1648.
In Jedburgh Parish, there was an Andrew Rae with a son named William, born February 22, 1666.
So, it looks like there were Reas in Roxburghshire from as early as the 1640s. Now you know who you're possibly descended from, you just have to find the exact links.