Is ceviche good the next day?

Only add veggies to the ceviche you are going to eat. The leftovers, without veghies will be good the folliwing day.
Drain the ceviche, then add orange juice and olive oil toss.

It won't poison you, but it will be quite unappetizing because the acid that "cooks" the fish will turn it into mush when left that long. Eat it up.

You definitely won't be poisoned if you refrigerate it. The only thing that happens is that the acid will over "cook" the fish and scallops. They may be a little rubbery. I wonder if draining it would help? It might be worth a try. I would pour the ceviche into a mesh strainer and set the strainer over a bowl and lay plastic wrap or damp paper towels over the strainer.

As long as you keep it very cold, it will be fine for one more day. I would put the bowl containing your ceviche inside a larger bowl filled with ice, cover both tightly with a layer or two of plastic wrap. Put in a cold fridge. You may find you need to squeeze more lime, lemon or orange over it tomorrow to brighten the flavor again.