I don't think the Watch were used for colony sites I thought it was Merchants that found Colleen and Merchants are likely the only caste with the freedom (under the watchful eye of a Warrior minder) and incentive to find new habitable systems. In my opinion the Spirits took the Mandrill Watch over at the Loremaster's request in exchange for support of the Kindraa in the event of an attack and to use them as an alternate intermediaries to the Sharks for trade reasons. They also probably received aid in developing weapon designs and expanding factories from the Mandrills which were undergoing a bit of an expansion before being kicked down during Reaving.