No, the last version of OSX that supports PPC systems is 10.5 Leopard.
Your Mac Mini should have no problem triple booting the three OS' you've mentioned.
I haven't tried to do that recently, but I'd start by setting it up for dual booting (with OSX and MorphOS), the library has an article on that.

Then add Linux.

With the added qualifier that I'd probably set up the partitions for all three before setting up OSX (you can always change the file system they are set up for later).

BTW, isn't the AresOne PPC basically the same as the SAM460 you already own?