A 7rs would be ok for this, Personally i like to use a little more voltage when shading & colouring...i normally run my liners at about 7-8volts,shader's at about 8-9volts,& same with colour......its just a matter of personal preference & how your machines are set up...if the colour is not going in,just up the voltage a little......just experiment on some piggy...hope this helps..
You can use either 7rs or 9rs, why not try it first on some pig skin and just experiment. The same goes for power settings, you will probably find that different machines will need more or less power so it's a good idea to tune them without too much emphasis on your power setting. So long as you are around 7 - 9 it should be fine, I have found by trial and error that it really depends on what machine and power pack I use. One of my liner machines seems to run best at 7v the other one which is identical runs best at 8v, they both have 10 wrap coils which makes them run a bit smoother than they did with 8 wrap. I hope this info helps, but as I've come to realise it's all about practice and then more practice. Keep it up mate and post the picture when you have finished it:thumbsup: