usb ports on MacPro

No problems.

Each port makes its own chain of devices that stay separate. There will be no overlap and no confusion.

Each port has its own POWER budget, so self-powered Hubs and devices are preferable for long term trouble-free operation.

USB-2 type A has four conductors:

To get to USB-3 speeds, more conductors are added as a superset, using balanced drivers and receivers and full duplex operation:

so if all you ever want is USB-2 speeds, your old cables are fine.

If you ever want USB-3 speeds, all the USB-3 devides need to be contiguous next to the computer port, because the first USB-2 device/cable will slow all further USB devices down (it does not conduct the extra signals through.)

Yes, indeed there IS a USB-3 extension cable. It has a blue insert and 9 conductors all the way through to a blue insert at the distal end.

Your older cables (4 conductors, no blue insert) can not provide USB-3 speeds.