Increasing the number of unicorn workers to suit your CPU and RAM capacity is perfectly reasonable. The “two unicorns per core” guideline is a starting figure. CPUs differ (wildly) in their performance, and VPSes make that even more complicated (because you can never tell who else is on the box and what they’re doing with the CPU), so you start conservative, and if you find that you’re running out of unicorns before you’re running out of CPU and RAM, then you just keep increasing the unicorns.
As far as disk cache goes, you need as much as you need. As you increase RAM consumption, keep an eye on your IOPS graphs, and particularly the percentage utilisation of the disks (what sysstat refers to as %util). There’s two points you want to be aware of: when your read IOPS start to stay persistently higher (that means that the working set of disk pages no longer fits in memory – you may or may not have hit that already), and when peaks in disk utilisation start to get close to 100%. The former tells you when your RAM consumption is starting to impact performance, and the latter tells you when you’re starting to saturate the disks. You want to consume RAM up to the first point, and you can drive it as far as you’re comfortable towards the second point, depending on your tolerance for performance degradation (keep an eye on your service times!).
Another RAM-related metric to keep an eye on is swap rate. That’s not how much swap space is being used (as long as your swap partition isn’t full, it doesn’t matter), but the number of pages per second being written to and read from swap. Swap writes are fine, but if the system is constantly swapping pages in and out, even only a few per second, you probably want to back off on your RAM usage a bit.
Just to keep you on your toes, swap activity counts towards disk IOPS, too, so your disk utilisation will likely go loco bananas when you start to run out of RAM, due to both extra disk reads (because the cache isn’t big enough) and increased swap activity, because the working set of memory pages doesn’t fit any more. That’s a recipe for performance disaster right there.