Are there any rules to using "semi-", the prefix?

Michael Quinion, Ologies and Isms: Word Beginnings and Endings (Oxford, 2002) has this to say about the prefix semi-:

semi- Half, a part. {Latin semi-, half.}

Th strict sense of a half occurs only in a minority of words, of which examples are semicircle; semidiameter; semilunar; shaped like a half-moon or crescent; and semiquaver, in British musical terminology a note having the time value of half a quaver, a sixteenth note. A few terms extend the idea to that of occurring twice in some time period, as in semi-annual, occurring twice a year (nominally every half year), and the North American semi-monthly, occurring or publishing twice a month (or every half month). A related idea occurs in semi-final, a match or round immediately preceding the final, the 'half-final'.

Most terms in the prefix, however, signal that something is partially or incompletely so: semi-professional, semi-conscious, semi-retired, semi-literate, semi-skilled, semi-derelict, semiprecious, semiconducting (of a substance that has a conductivity between that of an insulator and that of most metals), and semipermeable (of a material or membrane that allows certain substances to pass through it but not others).

A similar analysis could be applied to English words that begin with half-: there are the exact one-half words (half-crown, half-dollar, halfmoon, half-hour), the approximately one-half words (half-cell, half-life, half-mast, halftone), and the the words where half- really just means partial (half-baked, half-cocked, halfhearted, half-light, half-truth).

Under the circumstances, I have trouble drawing a bright line between semi-skilled in "a semi-skilled position" and semi-sales in "a semi-sales position" and saying that the former constitutes a legitimate use of semi- but the latter does not.

"Semi-sales" is a bit off normal usage and ambiguous. It's not a term I hear. However, context is important. It might work in a more informal conversation where there is more shared understanding so words can be used more flexibly. The fact that it didn't work here suggests that it might be a step to far in the neologism direction with this person at least.

In more formal usage, like a job description, it would be better to be more explicit, eg, "part sales, part technical support", "sales is part of this role", or something along those lines.

Semi tends to be used in situations where there is a complete object or a final state, like semi-circle or semi-final. In the case of an ongoing activity like sales it's not quite clear what the complete state is. In comparison, semi-skilled implies that the person in the position has a partial or incomplete skill and could in theory become fully skilled. That said, if everyone used it...