I am assuming they were all born in Llangurig parish given their appointments as parish clerks.
Don't assume anything.
Does the approximate birth year of Edward (c1771) come from the 1841 census? If so, expect a rounding variance of up to five years.
It is equally possible that the 1768 Edward REES was born, married, and had a son Edward elsewhere (in the county?) and moved to Llangurig in or shortly before 1768. He might even have arrived to take up a schoolmaster post and doubled as parish clerk.
I don't know the historical population of Llangurig parish, but according to Wiki the population of the village in 2011 was only 723. Schoolteachers in small villages were generally poorly paid. Their level of education varied, but it would be unlikely for a village schoolmaster to have had either a public school or university education, or to come from a family with much money. He would not be an attractive match for an heiress.
My suggestion would be to consider:
1) Did Edward (1768) REES have any children baptised locally?
2) Is there a burial entry for a wife or widow of Edward (1768) REES?
3) Are there any burials for young children of Edward (1768) REES?
If a wife can be identified this could help trace a marriage, and if there was an Edward son of Edward baptised in the same parish as the marriage then the trail could heat up.
Philip