Couple of ways to do this in Particular.
If the particle is a still, put all 26 particles in a precomp, make their duration 1 frame and sequence them with no overlap. Change the precomp to 26 frames long. In your main comp under particle choose sprite or textured polygon(polygon has more options, but it depends what you need to do) and choose your precomp as the texture. Change the time sampling to Random – Still Frame.
If you are working with 26 animations, you would need to precomp them also and sequence. But instead of “Random – Still Frame” you would chose “Split Clip – ?(depends on your animations) and change the Number of clips to 26.
Johnny Cuevas, Editor