Limits on using new words in scrabble
Let's check the Hasbro rules (http://scrabble.hasbro.com).
It's not verbose but it can be inferred from the text.
Rules are different for the first play and for subsequent plays.
Bold is mine.


  
  The first player combines two or more of his or her letters to form a word and [...].
  


Note that the rules for the first play say to form "a word", not "words". So one word is inferred.


  
  Play passes to the left. The second player, and then each in turn, adds one or more letteres to those already played to form new words.
  All letters played on a turn must be placed in one row across or down
  the board, to form at least one complete word. If, at the same time,
  they touch other letters in adjacent rows, those must also form
  complete words, crossword fashion, with all such letters. The player
  gets full credit for all words formed or modified on his or her turn.
  


Note that letters must be placed in one row to form a "complete word", "crossword fashion". So it's not explicit but to form complete words in crossword fashion seems to imply that subsets of words are not allowed (in crosswords each row across or down is a single word).