From my experience, the gravels will mix and the smallest grains fall to the bottom and the larger will remain on top. I notice this even with my same-sized gravels and Flourite which has some very small sand-like particles and some 1-2 mm sized particles; the smaller are at the bottom, always. You can clearly see this along the front glass.
We sometimes forget that in a healthy balanced aquarim, water is constantly flowing down through the substrate and back up again, heated in the substrate by the bacterial processes occurring there. This convection or thermal flow is I suspect why the substrate shifts. As it is not static, the otherwise-logical mix of small/larger grains does not occur. And of course, there are plant roots burrowing through, and perhaps Malaysian Livebearing snails; both of these cause substrate shifting, which is why they are so useful.
I would replace the pea gravel with either fine gravel (1-2 mm grains) or coarse sand. As someone mentioned, two types of substrate mixed usually does not look "natural" but like two things mixed.
If the catfish are Corydoras, fine gravel works well, or sand. Except for the dwarf species which seem to do better over sand, which probably is not surprising.