Assuming that you already have a blade on you, for me the next important tool in a FAK would be scissors. Although the scissors on a SAK are good, I'd buy a dedicated pair of quality rescue scissors for a FAK.
To add a SAK for a pair of backup scissors, I'd recommend the Ambassador for a SAK. It is a bit larger than the Classic, comes with an extra small blade, but still thin and light.
To add a SAK for a pair of backup scissors, I'd recommend the Ambassador for a SAK. It is a bit larger than the Classic, comes with an extra small blade, but still thin and light.