well i'm using hipshot bridges and tuners for many years so I can help
- .125 or .175 is the difference in height, you may need a .175 if you have some neck angle for exemple or if you need a heigher action... .125 works great if your fingerboard is flush with the body for exemple...
- to install its very easy, hipshot recommends that the attachement screws must be placed 2cm back from the intonation point, however I found out the 2.2cm is much better and gives you more intonation range...
- I dont think you can directly install a hipshot on your Rg7321 without having to drill some new holes
