Hello,
from the term switching spikes I assume, you are complaining chiefly about higher frequency components and not the switcher fundamental. This should be analyzed carefully cause different interferences can require different means to overcome them.
Higher frequency interferences ore "spikes" can be most effectly eliminated with combinations of small inductances (can be ferrite beads with sufficient current capacity, ferrite or air chokes) and ceramic capacitors ranging from a few to several ten uF. Low ESR input and output capacitors as the suggested polymer aluminium type additional reduce ripple at fundamental frequency. Or use higher frequency switcher and all ceramic capacitor design if you're ready to face the challenge.
A "solid ground" is also required for effective filtering. But a SMPS placed on a common ground with the rest of the circuit could also inject interferences into the ground plane. Thus it can me meaningfull to have a separated power ground for either all or each individual SMPS tied to common ground at one point.
The important thing is no to have the switching currents, commutating from input capacitor and high-side switch to diode/low-side switch flowing through the common ground. A first LC filter should be placed at the interface of power ground related circuit then.
Regards,
Frank