It's fine to include an occasional single-word entry in the stacks you describe. Lively words work best; Conversational words are also acceptable. Less desirable words might contain lifeless prefixes and suffixes like PRE-, MULTI-, INTER-, -NESS, -OUSLY, -ENCE, -MENT, etc. Technical words are discouraged.
I would probably limit the single-word entries to one in a stack of three and very rarely two in a stack of four.