Basically I'm trying to figure out how Sony managed to make the PlayStation when 3D hardware was mostly limited to SGI's UNIX machines like the Indy. The N64 was co-developed with SGI, and at one point SGI even approached SEGA USA(Sega Japan told them to kick rocks apparently). The PlayStation even uses MIPS processors(which is what SGI used on their $100,000 graphics workstations), but I can't find any real connection between Sony and SGI.
People allege that Ken Kutagari(designed Sony's DSP/sound processor that was used on the SNES) was also responsible for the PS1's GTE & GPU designs because he was allegedly the driving factor of the System-G.
Designing hardware for real-time 3D graphics was not a problem. Silicon Graphics may have been most famous, but they weren't the first, nor were they the only one. The problem was making it cheap enough to put into a $300 consumer device. Sony's solution was to use much cruder, and thus cheaper, hardware than was used in the workstation market, and selling at a loss until Moore's law dropped manufacturing costs.
Playstation using a MIPS CPU is not a big mystery. Anyone could license the architecture, and if you wanted a medium-to-high-performance custom processor without rolling your own, there weren't a whole lot of alternatives. Sony also had experience with MIPS from their workstations.
Kutaragi was the system architect for the PS1, I'm not aware that he was involved in the design of any of the custom chips in any kind of hands-on capacity.