Will other devices interfere with programming a ATmega32U4 if attached to SPI bus?
As per AVR042: "AVR Hardware Design Considerations", subsection 4.1.1, "Shared Use of SPI Programming Lines":


  If additional devices are connected to the ISP lines, the programmer must be protected from any device
  that may try to drive the lines, other than the AVR. This is important with the SPI bus, as it is similar to the
  ISP interface. Applying series resistors on the SPI lines, as depicted in [figure 4-2], is the easiest way to achieve this. Typically, the resistor value R can be of 330Ω.


See AVR042 for further details and considerations.