First, actually Linux and Minix (and BSD as far as I know) are both POSIX compliant, so the syscall table will be more or less the same. The thing that changes between architecture is the instruction set, which is different, no matter what. That's actually the point of POSIX compliance: to define *nix-like systems in a way that is predictable.

Second, actually the more architectures you are compatible with, the more code is involved, and it tends to be more, not less, complicated. This is especially the case because all OSs have some assembly in them, and that assembly DOES change per your architecture.

Third, Minix is a "working" kernel. The main difference between Minix and BSD/Linux/whatever is that it is a microkernel, which is easily the best to learn on, on account of things like the permissions structure being MUCH simpler (arguably one of the most difficult things to grasp), but not the best in terms of (for example) security.