only one that I can find that fits more than one of the facts is this
1901 Derwen Fawr Cottages, Blaenhonddan RG13/5062 folio 77 pg 42
James Rowlands head mar 36 colliery weigher b. Pembs, Whitchurch
Mary wife 37 b. Glam, Cadoxton
Thomas son 7 b. Melincryddan
William son 4 b. Cilfrew
Annie Mary dau 4 months b. Cilfrew
Could marriage cert say 21 not 27? Are there are Rowlands witnesses on it to help?
Strangely, this is the family I found searching on Merthyr in 1891. The only other James Rowlands coming up with residence of Merthyr was 14.
1891 1 Cynon Row, Cwmdare RG12/4443 folio 35 pg 21
John John head mar 34 coal miner b. Glam, Cadoxton
Ann wife 32 b. Llangeler, Carms
Wiliiam John John son 9 scholar b. Ton(?) Ystrad, Rhondda Valley
Rachel Ann dau 2 b. Penrhiwceibr, Mountain Ash
David Thomas son 5 b. Penrhiwceibr, Mountain Ash
James Rowlands boarder mar 26 coal miner b. Whitchurch, Pembs
Mary Rowlands boarder mar 28 b. Cadoxton, Glam.
If it is this family, then this looks like the birth registration for Thomas
Thomas James Rowlands 1Q 1894 Neath vol 11a pg 831
There is a birth reg in 1888 for a Thomas Rowlands 2Q 1888 Merthyr Tydfil vol 11a pg 590 and 4 more Thomas Rowlands registered in Pontypridd area in 1899 if you want more choices, but they do not appear on the census with father called James as far as I can see. You could always order all the birth certs with a check of the father's name if you're getting desperate - refs for the ones in Pontypridd are on FreeBmd.