Merging two seperate companies SEMP into one

I have been tasked to merge two seperate company SEPM's into one centrally manged solution. There are a number of factors in play for this decision but the basics are that I have an SEPM v 12.1.6 located in our data centre. The data centre is centrally available to both companies as the Domains are merged. Both older SEPM versions are 12.4

Another consideration are the older clients out there - running version 11.x (not supported since Jan 2015 with updates) and a number of older OS versions out there (XP).

Can someone point me to a best practices article that will enable me to:

• consolidate both existing into one central managed solution
• push updated clients (12.1.6 at all 12.x previous clients)
• find the old SEP version 11 clients
• a process for putting a GUP at larger sites
• a process for managing non domain sites for updates - local workgroup installations at smaller locations

Thanks!