Other than adding a small amount of weight toward the back end of your design, adding pods on fin tips should not degrade the stability of your rocket. It might actually help it.
There is a phenomenon called a tip vortex that occurs at and around the outboard edge of your fin. (Tip vortices are more commonly discussed in connection to aircraft wing tips, but the same idea applies to rocket fins.) When your rocket is deflected from the ideal flight path, it is travelling one way but pointing another (momentarily). That is when your fins go to work. They are effectively turned to an angle of attack, measured from the plane of the fin to the path of the rocket's flight. In this condition the fins act as little wings to try to pull your rocket's back end back in line with the flight path. When they generate lift, they also create tip vortices.
This vortex is easy to understand when you think about the (relatively) higher air pressure on the 'bottom' side of the fin/wing and the (relatively) lower air pressure on the 'upper' side. At the tip, these pressures try to equalize by introducing a swirl, or vortex, around the outer edge of the fin from the high pressure side to the low pressure side.
The energy that is spent in the process of creating this vortex comes from your rocket. The energy is lost in the form of an increase in the aerodynamic drag. So, we really don't want any more vorticity than we have to have.
Fin tip plates (and end pods) act as a barrier to the creation of tip vortices. They act to stop (or greatly reduce) the tip swirl and therefore reduce the drag. In stopping the tip vortex, these end plates have the effect of making the fin more aerodynamically effective. Your rocket should actually become slightly more stable with the same size fins.
If you are using tip pods, some part of the pod will also contribute to the effective fin area. It is easy to visualize a fin-with-tip-pod at some severe angle of attack, and to imagine that the flow over the outboard half of the pod would just roll on across that outer surface. The airflow that strikes the inboard half of the pod, however, tries to move slightly inboard but is sort of 'trapped' by the fin; the inboard half of the pod effectively acts as fin area.
Did any of that make sense?
