I seriously doubt China will base the Zubrs on any island in the SCS.
It would be too blatant a threat and escalation for China to choose to do unless something drastic happens. And even then I would not really expect Zubrs since they just don't fit any military role the PLAN might realistically be ask to do. I mean what plausible target in the SCS would require a Zubr full or marines and equipment to deal with?
If push really comes to shove, the PLAN would be the first responders, and after they are finished blasting whatever target offended China, a RIB full of their standard shipboard compliment of marines would probably be sufficient to mop up what's left and plan a flag.
Point is, any target that needs a Zubr full of marines to deal full cannot be dealt with by a single Zubr. You will need serious fire support to soften up such targets before you will even start to think about a beach assault, and that kind of firepower will need substantial built up anyways, so what benefit do you really get out of forward deploying Zubrs?
Doing so will only present a big fat juicy target to potential enemies and also limit the flexibility you have to respond to unexpected developments elsewhere compared to just keeping them based fairly close to the mainland.
I think that the main reason for the Zubr deal now is less the operational capacity the Zubrs themselves offer, and more to do with acquiring the technology and know-how to build them. The Zubrs bought could just be the price the Chinese had to pay to get that technology.
I would expect the PLAN to play around with the technology and probably start building Zubr based test ships for purposes other than the traditional beach assault mission the Zubrs were designed for. So, imagine if they removed the troop carrying capacity and stuff some big AShMs in a Zubr hull and made a bigger, badder and faster Type 022 missile boat.
They could maybe develop an ASW variant since the speed and zero draft of the Zubrs would make them near immune to torpedoes. You may need to give such a variant a hanger and helipad and/or use another asset to spot for it on account of the noise from the Zubrs.
Those are just a couple of examples I just thought off ottomh, but I think it illustrates the point that the Zubr bass potentially to be applied to other purposes and missions.
The reason for the Chinese starting to build Zubrs so soon could be to give the shipyard and workers some hands on experience applying the skills and technology they just acquired, and also as a test case to see if they are ready to start moving onto other designs.