You can use any kind of input jack you want, they just assume you will use RCA phono-type jacks since that is pretty standard. If you want to use a minijack, I'd still put RCA's on the PPA but make a minijack adapter cable w/some nice wire.
I read that but i don't understand what it says about the R8 maybe someone can put it in simpler terms
This is the "source resistor" for the op-amp's class A biasing cascode. See this section for details of how this works.
To go through the value picking procedure described in that section, you will probably need a good selection of values in the 10Ω to 1KΩ range. You can use cheap 5% carbon resistors here, and just get a few of each of the E24 values in that range. (10, 11, 12, 13, 15, 16, 18, 20 ohms...) If you really wanted to be frugal, you could limit the range to the 47Ω to 470Ω range. You may also consider getting a 5% resistor selection kit, which many manufacturers offer.
Optional? Yes. You can jumper it in some situations if you're using the cascodes, and if you're not using the cascodes you should leave it empty.
When setting up the cascode, solder the JFETs in first, leaving both R8 and R9 empty. Turn the amp on, put a milliammeter across R9 and then touch varying resistor values into the R8 position until the current draw across R9 is where you want it. Somewhere around 1mA is a good target value, but you might try different values to see if you can hear a difference in the headphones. Once you find an R8 value you like, solder it down and add R9