So I can use any SITE that covers my area to assign it the SPECIFIC TALK GROUPS i want if they are in that area as well?
You cannot "assign" talkgroups to a specific site, and the RR database does not include any information on which site (of a multi-site system) carries which specific talkgroups.
In your programming, include the site (or sites) that cover the area where your talkgroups of interest are located. If one of your talkgroups is in use on one of your programmed sites, you'll hear it, but there is no way to actually 'assign' any TGID to a certain site, other than creating a system, with only the desired site, and the talkgroups of interest. But as long as you are using ID Scan, you'll only hear the talkgroups you want, regardless of which specific site that is carrying them.