I don't think "police" as the term exists now is something that should be required for Asgardia, although this could be just my optimistic side that thinks with suitable education and being provisoned with requirements would vastly eliminate the causes for "crime".
I do see place for some sort of "security forces", however, far more prominent from establishing these would be to address the failings in pretty much all existing examples of such in order that what results from isn't subjected to the same flaws(generally at the human element) that permeate such corporate structures.
Even more prominent that establishing a police force, would be to establish the very policies these officers are intended to enforce - as much as the options open to them in enforcing it.
Also, contrary to your statements, the greatest threat to Asgardia in the short and medium term will indeed be from Earth, with almost everything else being able to be accounted for simply by sensible planning/design. Physically, digitally, and politically there is much threat to found from Earth. Whilst the larger percentage of Asgardians are on the surface, they are ultimately physically succeptable to the nation in which they reside and protection from this will be reasonably difficult. When it comes to orbital hardware, then most nations on the surface have the ability to strike at orbit. All it takes is the inclination.