I want to create a slave site using a different template from the master site. Do I simply go to public_html/igu and extract the quickstart package even tho it includes a joomla installation?

In general a quickstart template contain a Joomla including the joomla template and some data with modules and plugins.

If you want to create a slave site using the template that is present in this quickstart, you can not unzip the quickstart as the concept is to share the same Joomla files and folder than the master and not have a duplicated joomla.
You have to install the template as any template in Joomla extension installation.
This will not give you the whole "quickstart" as you will not have the content of this quickstart package.

If you want to redo the same as the quickstart template, you will also have to install all the modules and extension in the master website before creating a slave site that will use it.

ok, thank you.