What, if any, naming convention was used for the standard Unix commands?

Historically, UNIX commands are short because, at the time the OS was created, memory was scarce, networks were slow, keyboard were hard to use, and terminals (when available -- most of the times you got the output as a paper print) had small resolution. So it made sense to try to economise as much as possible.

Clearly one couldn't shorten at maximum any arbitrary command and yours is a good example: md could better be attributed to a command to generate a MD hash, and a shorthand for "directory" is "dir", so choosing mkdir for a command that creates a directory makes perfect sense.

See also my answer here: Why are UNIX/POSIX system call namings so illegible?

To sum up, there is no convention - as far as I know - for UNIX command names, apart from the guidelines above mentioned which come from historical technical limitations.