I reckon it is a bitter different from that because a Marine is qualitatively better than a Explorer or Acolyte. For a start, Marines have about double wounds, Unnatural Strength and Toughness, better gear (Astarte boltguns will mince an Explorer in one shot nearly) and power armour. That alone means that a single Marine is more than a match for a couple of Rank 5 Explorers. An Acolyte or Explorer will only get a max of around 20 wounds over the course of a career and only the very few will get Unnatural Strength or Toughness. A rank 5 Explorer will certainly have more skills but when it comes down to combat (which is where a Deathwatch Marine will always reign), a rank 1 Marine will outclass almost anything.
Having said that, there is real scope for a Kill-squad being supported by higher level Explorers (atleast Rank 5) and Acolytes (Rank 8) and Throne Agents, especially for less combat orientated missions.