[Samba] Domain SID vs. Local SID on Domain Controller & SID requirements

[Linda Walsh]

IF a samba server is setup to be a domain controller, should
it's local SID = the domain SID?

Also, what are the requirements of a SID?

I usually see S-1-5-21-x-y-z, where x,y,z = 10 digits, but
could x,y,z be 1,2,3 (for example)?   I.e. do they have to be
10 digit numbers or can they be shorter? 

If I have a simple setup, and want a sid I can remember can I
just make it 'short'?