What will have a bearing on this is the number of partitions you are currently using (including any recovery partition).
As you have already Bootcamped your system you are effectively limited to 4 partitions (without some manual tinkering). Without a BC partition (i.e. still GPT only) you could have as many partitions and OSes as you want.
Some Linux systems can be installed via EFI (like Ubuntu) though this can be a little more convoluted to do.
I would put it on my 2nd drive along side windows
Linux can be booted from external drives. You should be good.
Thats the best solution.
Be warned: Installing Linux on a mac is rather high difficulty and can cause disruptions in your system.
This said:
You can use an app called reFit to do this.
I installed windows and ubuntu on my macmini this way.
I recommend installing windows first, then linux.
Instructions are somehwere on the internet.
Again: Its rather tricky. Only do this if you really need to and know what you are doing.