The only movement between SIA and SQC is when pilots from SIA who are reaching the retiring age join SQC. Used to be guys reaching 62, who will move to SQC and fly till they are 65. Not sure of the policy in SIA now. Another movement will be First Officers from SIA who hold CPLs but no ATPLs (hence, not being able to become Captains) moving to SQC.
Newbies go to all subsidiaries (SIA, SQC and MI).