USIMs are physically the same, they just have some slightly different software on them. The Nexus will use a USIM just fine.I know for phones you need a Japanese address, and they use something called usim. It doesn't seem to look any different to a regular sim, but my Japanese phone only reads that (my optus sim is region incompatible). I have no idea if this is helpful, since you should be able to buy a sim from a convenience store. If you can get it, softbank is probably the best company if you're just calling people within Japan. Something like 300¥ for unlimited texting on prepaid.
To make things easier, there's companies like eConnect Japan that will have the SIM delivered to your hotel (or picked up at the airport in some situations.) No idea how their rates compare.