As far as I know, you can only install one version of IIS on Server 2012, which is IIS 8.
I understand that you would like to use IIS6, but there shouldn't be anything you can do in IIS 6 that you can't in IIS 8, with a little configuration. Normally things such as enabling 32-bit support for external DLLs, changing application pool identity, Legacy SMTP, Legacy FTP support, ASP settings, etc. Also IIS6 is not great a security point of view.
IIS8 comes with management tools for IIS6 for configuring legacy applications that use services such as SMTP.
You can use the IIS 6 tools by running
%windir%\system32\inetsrv\InetMgr6.exe from a CMD in server core or Administrative Tools - Internet Information Services (IIS) 6.0 Manager in Desktop Experience
To install a full IIS 8 including all features, you can use the command below.
Install-WindowsFeature -Name Web-Server -IncludeAllSubFeature
If you are able to update your question, to include what server version you are running (Core, Desktop Exp) and the reasoning why you need IIS6, It will be easier to point you in the right direction