According to the "African Court Calendars" that I have access to (some missing), the "Thomas" was in Port with these as captains:
1813: A Landele (Mauritius to London) & C. Allen (Mauritius to Cape)
1817: William Scott (Mauritius to Cape) & G. Jackson (Downs to Cape)
1819: W. Scott (Rio de Janeiro to Cape) - arrived 20th February 1819
[There is also a listing for a "Thomas", Robert Cornfoot, which arrived on 10 January 1819 from Portsmouth & St Jago to the Cape which departed again on 30 March, and it appears again in 1822, with Richard Winspear as Captain, from Portsmouth & St Jago to the Cape].
The Harmony was wrecked on 13 March 1826 so I checked when Butler would have become Master of the Thomas and found these:
"Cape Town Gazette & African Advertiser", Friday, March 17th, 1826:
Arrived Table Bay 17th March, Thomas, English Schooner, J. Finnis, Master, from St Helena 7th Feb
"Cape Town Gazette & African Advertiser", Friday, 5 May 1826:
Departed Table Bay 2nd May, Thomas, English Schooner, R. Butler, Master, for the Mauritius.
Hope this helps
Sandy