Sixth = 6th
That is the correct spelling of "sixth" (the ordinal number 6th).
The ordinal number for 26 is 26th in digits. Spelled out, it's twenty-sixth.
They are: 26th and twenty-sixth, respectively.
The ordinal number for 26 is twenty sixth of 26th and that of 51 is fifty first of 51st.
Expressed in words, the equivalent ordinal number for the cardinal number 26 would be twenty-sixth.
The ordinal number 86th is spelled eighty-sixth.
Yes - cardinal numbers are the counting numbers. This is as distinct from ordinal numbers - the corresponding ordinal number for 66 is sixty-sixth.