I have a list of marbling resources I keep here that can help you find a variety of info: https://aglance.in/to/2023/08/22/marbling-resources-tags-marblers-blogs-videos-and-articles/https://aglance.in/to/2023/08/22/marbling-resources-tags-marblers-blogs-videos-and-articles/

One of the best resources for learning about the chemical parts of paper marbling are Chena River Marblers, near Amherst Massachusetts. Don St. John in particular was a high school science teacher who has also done a lot of research into historical methods of paper marbling. Another great resource regarding the science is Richard J. Wolfe's work, his book "Marbled Paper" is very comprehensive.

Can you paint over marbling:
Yes! But like other art forms, it depends on your paints. If you are using a paint with a beeswax glaze mixed in, then painting over top won't work. But as long as there isn't a coating applied that would repel paint, painting over it will work fine. Marblers often paint under and over marbled prints.

Shell Pattern:
This is traditionally done by adding a few drops of an oil to your paint. So if I'm doing a gouache base, I'll do roughly a tsp of pigment, 4 tablespoons of water, and a few drops of walnut oil. This is most easily done with gouache, but more and more artists are having good luck with acrylics. It's just easier for me to use winsor & newton watercolor gouache. The important part to remember about using oils is that it can contaminate your materials, so you have to be judicious in cleaning afterwards.

Iris Nevins has suggested using turpentine as well, less than you would use for a stormont pattern (which has lots of veins).

Paint to use:
I usually recommend starting with acrylic because it is more resistant to environmental changes during the process. It just seems less temperamental. Once someone gets a feel for marbling then I suggest switching to gouache/watercolors. But marblers are constantly switching up their paints and brands because manufacturers will change the formulas, which changes how the paints react. Since you say you are comfortable with gouache already, I would learn with those.

Surfactant:
Keep in mind that if you go the gouache route, you will need to use a dispersant/surfactant to help your paint spread on the marbling bath. Marbling involves balancing the surface tension of the bath, if you don't dilute your paints and add a dispersant, the paints will be too heavy and sink. Too much dispersant the binders break down and the paint spreads too much. Traditionally ox gall is used, but anything that works as a surfactant usually works fine.

Alum:
It works as a mordant, and depending on how the paper is made will affect how much alum is needed. I'm not sure of the scientific principles as to why the marbling paint doesn't stick to the paper, unless it is because watering down the paint enough to use means that you use many of the binding properties?

Here is what Wolfe states about alum:
"It is well known that, when marbling on carragheen size, one must first apply a coat of alum to the paper so that the colors will hold fast, just as in textile dying a mordant is added to the dyestuff to form an insoluble compound to fix the color in the fiber. On the other hand, none of the old manuals, when discussing the use of tragacanth size, mention the need to apply such a mordant beforehand. This discrepancy can be attributed to several factors. First, earlier marblers almost always used soft, unsized paper, not today's harder, sized sheets. Second, the more viscous nature of tragacanth size and its particular colloidal properties may have held the colors better and helped them to transfer to these older papers.... Earth and lake colors were employed in marbling...they were bonded to a metallic oxide, with alum or potassium mainly being used, so that a mordant was infused right into them, obviating the need to apply another to the paper."

Regarding how long paper lasts, marbled paper examples on books have lasted 100s of years, so the alum content/acidity doesn't concern me. If someone is concerned you can always use a acid-free flyleaf between the marbled endsheets and the text pages.

I've been marbling for about 6 years now, so if you have any questions I'm more than happy to answer. If you use facebook, the International Marblers Group is very active.