I have 16gb of RAM, but only 10gb are usable?

As I read the memory information you posted I don't see any problem.

My suggestion is to stop looking at the RAM and allow OS X to handle it.

The output from this terminal command will tell you how much memory is recognized by your system, copy and paste the entirety including the comma apostrophe at the end :

system_profiler SPHardwareDataType SPMemoryDataType | perl -ne '

$Memory = $1 if /Memory:\s+(\d+\s+\S+)/;

$Phase2 = 1 if /Memory:\s*$/;

$MemSpeed = $1 if $Phase2 && /Speed:\s+(\d+\s+\S+)/;

$Type = $1 if $Phase2 && /Type:\s+(.*)/;

END { printf("%s\n%s x %s %s\n%s %s %s\n",

$SerialNumber,

$Processors, $Speed, $Name,

$Memory, $MemSpeed, $Type);

}

'

then hit the return key for the output.

Quite terminal

You can read up on how memory management has advanced since with the introduction of Mavericks, same applies going forward with Yosemite.

http://arstechnica.com/apple/2013/10/os-x-10-9/17/#compressed-memory

I would not waste my time with trying to micro manage memory usage.

Try reading http://macs.about.com/od/macoperatingsystems/fl/Understanding-Compressed-Memory- in-OS-X.htm