You can download/install the MMSIM releases from the downloads site, using InstallScape (you might need to download and install this first, the details are also on the downloads site - it is available alone or as part of the Lic+Config Utils package).
Yes it is possible to use a newer (e.g. MMSIM7.1) version of spectre with the IC61x release (or IC5141 for that matter).
Once installed, you will want to have spectre in your path in addition to the IC release tools; if you are using the IC61x tools then I don't think it will matter if MMSIM is in the path first or after the IC tools, but as you noted, if using IC5141, spectre is included in that release, so if you wanted to use a newer version of spectre, MMSIM would need to be in your search path earlier than the IC tools.
The path to add is <mmsim_installation_dir>/tools/bin, this should be sufficient to pick up spectre.
Hopefully this answers all your questions.