IGI searched for: Father: John Quilliam, Mother: Isabel Shimin
1. Elizabeth Quilliam Christening: 08 MAY 1758 Malew
2. Thomas Quilliam Christening: 30 DEC 1759 Malew
3. John Quilliam Christening: 26 FEB 1762 Malew
4. Jane Quilliam Christening: 08 JUL 1764 Malew
5. John Quilliam Christening: 13 JUL 1766 Malew
6. Catherine Quilliam Christening: 24 MAY 1770 Malew
7. Esther Quilliam Christening: 03 MAY 1772 Malew
8. Joney Quilliam Christening: 22 APR 1774 Malew
9. John Quilliam Christening: 12 JUL 1778 Malew

IGI searched for: Father: Wm Quilliam, Mother: Catherine Bell
1. John Quilliam Christening: 20 MAR 1765 Malew
2. Thomas Quilliam Christening: 21 SEP 1766 Malew
2. THOMAS QUILLIAM Christening: 28 SEP 1766 Malew
3. William Quilliam Christening: 10 JUL 1768 Malew
4t. David Quilliam Christening: 19 APR 1770 Malew
5t. Henry Quilliam Christening: 19 APR 1770 Malew
6. Henry Quilliam Christening: 16 FEB 1771 Malew
6. HENRY QUILLIAM Christening: 16 FEB 1772 Malew
7. Margaret Quilliam Christening: 19 SEP 1773 Malew
8. Elizabeth Quilliam Christening: 16 JUL 1775 Malew
9. Charles Quilliam Christening: 15 FEB 1778 Malew
9. CHARLES QUILLIAM Christening: 22 FEB 1778 Malew

of the 2 Malew Thomas Quilliams the one born in 1766 looks more promising as the other one was the eldest male child and would have inherited the farm and thus would have less incentive to emigrate.

To try for verification get a copy of the will for QUILLIAM, William 1801 Mal A 20 0106234 and see if Thomas is mentioned. If that doesn't work the 2 other Malew Thomas Quilliam wills (1789 & 1821 from Brian's list) would at least rule them out as suspects.