I currently have Revit 2015 and 2017 on my computer. I would like to bring a Revit 2015 model over to AECOsim using i-model. However, when I install i-model, the plug-in is only available for the Revit 2017 installation. Does this mean I will have to uninstall Revit 2017 in order for the installer to be "forced" to work on 2015
To take your Revit model to AECOsim you want to use ISM (Integrated Structural Modeling) and not i-model. The latest release is compatible with Revit 2017 but no longer supports 2015. Here is a link to the ISM wiki page.
For Revit you need to install both the ISM Revit Plugin and Structural Synchronizer. The Revit plugin adds the ISM commands to a Bentley tab on your Revit menu. Structural Synchronizer makes the whole thing work. There are download links for both apps on the wiki page.