Any surviving records will most likely be at West Glamorgan Archives.
For owning companies that were limited liability, there will probably be limited corporate records in The National Archives at Kew but these will be limited to company formation and structure, annual accounts, probably lists of shareholders, possibly annual reports by the board of directors, and company dissolution records.
Local newspapers (some are available on Welsh Newspapers Online down to the 1910s) will have reported events of importance. The best runs are held in West Glamorgan Archives, major libraries and the National Library of Wales at Aberystwyth.
The annual reports of HM Inspectors of Mines reported any fatal accidents.
The annual List of Mines is mostly available online at the British Geological Survey website. It lists owners and managers, and from the 1890s also lists total number of employees which is a useful guide to ups and downs of prosperity.
Abandonment plans are held by the Coal Authority.
Best wishes with your research!