hi,
It is possible to move multiple site collections to a single site collection as sub sites . You need to use stsadm -o export / import commands to do this.
It is not possible to move site collections along with databases as subsites.
So you need to choose one site collection and then the database associated to it and then follow stsadm -o export/import. Once you are sure about the data you can delete the site collections or detach those databases from central admin.
You have also another option, moving all site collections from multiple databases to a single database. You can achieve this by using stsadm mergecontentdb
http://technet.microsoft.com/en-us/library/cc825326%28office.12%29.aspx
you can also use batch site manager to move site collections from multiple databases to one db that comes with admin tool kit for sharepoint:
http://technet.microsoft.com/en-us/library/cc508852%28office.12%29.aspx
In my opinion moving these site collections is completely depend on how the security , permissions, sensitivity of the data is desined in those site collections.
Finally, Moving small site collections to a single database is my recommendation. ( you need to work hard on permissions if you choose export/import method)
Best Regards, Ashok Yadala
