The Polysentience is accessible to anyone with Mindware by default. Presumably no other race in SARP has the same kind of formats as the Freespacers do, so unless they have software compatible with the formats (which is highly unlikely), they wouldn't be able to access it.
The most compatible format in the Polysentience to any alien console or operating system would definitely be .asg (Audiovisual Sensory Graphic), which can be used through screens and manual interface outside of Mindware, but even then the software would have to be compatible with the format in order for it to be properly used without errors occurring.
I can't see any other hardware being able to use .rdf (Raw Data Format) or .vri (Virtual Reality Input), as these formats are directed and specifically programmed to use Mindware hardware to access particular areas of the brain as an interface.
Other than these physical restrictions, nothing stops anyone from accessing the Polysentience. The Freespacers are loathe to censor their networks from whoever wants to use them. And since Nepleslians now have legal, official access to Mindware implants (thanks to yours truly, Strangelove and Kokuten), Nepleslians can use the Mindware to access all formats types within the Polysentience to interact, communicate and entertain themselves with little to no problem.
tl;dr - It would only be accessible to compatible software. While .asg is fairly easy to translate to interface, .rdf and .vri formats would be almost impossible to access without mindware implants. Other than that, there's nothing stopping anyone.