From a technical perspective, deploying multiple roles on a single machine is simple, we do it on our development servers regularly.
I think the general recommendation, although I can't remember where it is written or who all recommends it, comes less from ArcGIS Server roles needing to be on their own machines/sites and more from segmenting various use patterns to different machines/sites. Even within a single ArcGIS Server role, there are times it makes sense to have separate sites. For example, deploying heavily-utilized geoprocessing services on a machine hosting map or feature services can lead to the gp services impacting the performance of other non-gp services.
For the situation you describe, I don't see a compelling reason to stand up a separate site. Since deploying ArcGIS Image Server on both the ArcGIS GIS Server machines will involve licensing more cores than you were originally planning, there could be an increased cost from an Esri licensing perspective. That said, deploying 3 Windows servers instead of 2 Windows servers comes with additional operating system licensing costs, not to mention the additional hosting cost of having a 3rd machine. What is the net change in cost? Likely not worth driving a deployment decision.
