I have the same questions as bnt83.
What's a horizontal indicator?
You can add a G5 ADI and HSI to any aircraft on the AML. Will it replace other equipment? That depends on what you'd like to interface to the G5s. Your autopilot and what it can "talk" to is the biggest factor there.
In my case I installed a G5 ADI and I'm hoping to install a G5 HSI sometime over the winter. I have to keep my old vacuum ADI which I placed where the turn coordinator used to be. The TC departed the aircraft unceremoniously to make way for the legacy ADI. I have an Altimatic III (Century) AP which is attitude-based. The G5 has no pickoffs for attitude and the only AP it will drive is a GFC500, so essentially my G5 ADI talks to nothing, really. This means I can't remove my vacuum system, for now.
When I add the HSI, I'll get rid of the vacuum DG. The HSI can be driven by a GTN or GNS series Navigator -- and I happen to have a GTN650, so I'm good. But if you don't have one of those, the HSI won't work. (That said, I have heard, but can't confirm, an IFD440 actually does drive the G5 successfully, but Garmin doesn't advertise it or support that installation.)
Hope this helps.