Spoiler for Your hypothesis ...cannot constitute a full rule since there is a small number of states with less than seven letters. It is true by pure coincidence (seven and last or second to last characters coinciding in states)
Why it contradicts the Game Rules: According to the game rules I was not allowed to use anything that does not appear in at least 50% of the cases. There are only 12 commands with less than 7 characters (4,5,6). So I cannot choose one rule for more than 7 characters and another rule for 6 characters or less - that would be cheating the rules. I must use the same deterministic rule for all cases.
BTW, If the situation were reversed, if one thought he had a rule for less than 7 or less than 8 characters, one could always try it on ABORT and see the result. Doesn't hurt
Spoiler for so...
Is your rule that you count 7 characters along, and if you hit the end you just alternate between the last 2 characters?