Can a squad use a leader or unit from another squad?
The Short answer is "yes, but throw in some other guys to make sure"

The only restriction that Warhammer 40k has for building armies is that the chosen models need to share a Faction Keyword, though specific factions within that keyword need to be grouped together. As it turns out, since Blood Angels and Dark Angels share two Faction Keywords, Imperium and Adeptus Astartes, that is fully acceptable. However, because they don't share all the same Keywords, they can't be played in the same group or Detachment. Though a unit may be played by itself at a penalty, it'd be polite to also gift a Troop choice, such as a tactical squad or scout squad, for each HQ Character you are gifting, like Sammael. Units like Apothecaries, Ancients and Champions are Elite Characters, not HQ Characters.

As it stands, having a chunk of the army that doesn't maybe hurt itself for loosing too many models, or takes moral checks, is a very good thing. Also, Dark Angels in particular tend to have particularly strong abilities to take advantage of. As of the current edition of 40k, 30k models that are listed in either the book that an army is tied to, or Codex, or approved through the Forgeworld website for the game may be used in Warhammer 40K armies. Such models include Tartaros Terminators and Contemptor Dreadnoughts