Unfortunately you can't read anything into the recurrence of the names Alexander, James, John and William, because these names are so common that there is hardly a family in Scotland in the 18th century which does not use one, more or all of these names.
Do you know where in Scotland your Joseph was born? Or the names of his parents?
There are five recorded Joseph Leslies born in Scotland between 1775 and 1785; there may also be unrecorded ones.
1. son of George Leslie, baptised in Rothes, Moray on 9 November 1775
2. son of James Leslie and Margaret Douglas, baptised in Rothes, Moray on 16 January 1776
3. son of George Leslie, baptised in Leochel Cushnie, Aberdeenshire, on 27 March 1781
4. son of John Leslie and Margaret Brown, baptised on 2 July 1785 in Duffus, Moray
5. son of Joseph Lesly, baptised in Leochel Cushnie, Aberdeenshire, on 27 June 1785