Well I generally find that "other breathers" method very ineffective as it takes a LOT of resources and time to build transports enough to move other breathers to every world that your own pop can't breath on.
I prefer leaving populations where they are and waiting until I have a large stable resource and research base then doing mass atmosphere conversion "blocks" in order to get my populations up.
As far as security troops I usually arm mine, I don't expect them to do much good against the enemy but I expect them to be able to crush rebellions without me needing to call in my "regular" troops, To keep my security forces available I organize them at HQ bases that are designed strictly to hold Security troops for deployment to various trouble spots around my Empire.
Especially since i Modded devnull to include base mounted space yards that produce at the equivalent of an 8 billion population world I usually build my troops at these bases and load rally battle fleets around them so that the battlefleet can pick up it's troops before leaving for the front.
For my "security forces" I do pretty much the same thing, I mobilize a fleet of small fast transports with a few warships to provide escort, then load up my security troops before having my security fleets head to the trouble spots.
You'd be suprised how quickly a squadron of superdreadnoughts escorting a fleet of 1,000 security troops can change the public oppinion in your favor
