Hey Darren!
First off I would generate a sine wave (make sure it peaks at 0) on the computer and run the signal to Mackie. Then adjust the input levels so the meter will never go above 0.
Now setting the output and monitor levels is a bit more complicated, because some equipment generates hiss...
You could also try:
1) Put MTR switch on DRIVE and adjust the gain so the VU meter reads max -10. (if thats not possible turn on pad on pre or microphone)
2) Now switch MTR to OUTPUT and turn output knob so the meter reads under 0.
3) Make sure Scarlett signal light never lights up red.
Hi MK!
Usually audio interfaces have preamps built in, so you won't need to buy it separately. Make sure that the interface has phantom power so you can work with condenser microphones like perception 220 (which is pretty good mic for starting out).
Any other things? XLR cable, mic stand, pop...
This is not a good idea. This will give you distorted picture of whats going on. (possible phase issues, comb filtering...) Use the Rokits for recording, Kustoms+Behringer for live sound.
We really don't know what you want to record. Maybe some mic's will help? :) Headphones, mic stands, cables...
This Samson is not a audio interface and recording via 3,5mm input is a good way to end up with bad recordings. Also usb mic's are not the way to go.
Try to find a decent interface for starters. M-Audio Fast track mk2 for example is cheap and helps you to get decent recordings.