Unfortunately you have hit the Welsh family history problem head on -
the paucity of surnames!
Both the surnames you seek are extremely common and to give you some
idea, in the 1851 census there were a total of 10 Thomas Williamses and
9 Margaret/Margret Thomases in the St David's, Fishguard and
Haverfordwest enumeration areas (excluding the town of Haverfordwest).
These areas probably represented a fifth to a quarter of Pembrokeshire's
population and Pembrokeshire's population was a twentieth (I'm guessing,
and will be corrected on this, no doubt) of Wales.
So, we're looking at a possible total in 1851 of a thousand of each name
in Wales within the age parameter you stated.