O, no MM!
You NEVER do a paella beforehand. At least a good paella. That's why when you ask for a paella you have to wait for about half an hour,
unless it is in the Menú del día, and so, you dont have to wait because they have freshly made paella all the time at lunch hour. You may have come to this conclusion because you probably had most times the one in the Menú (as I do).
A paella is usually made for at least two persons, unless in the Menú del día.
There are several kinds of paellas, the original one being (although this is not well known) the valencian one made of vegetables, rabbit (when avaliable) and chicken. Many times rabbit is substitued by other meats.
The most common (and the ones I like most) are fish and shellfish paellas, but I like the traditional one a lot. You
can ask for a fish paella because I think I heard that several times, specially if you go to a "paella only" restaurant, because they have all kind of paellas, like vegetarian paella (sometimes) - only vegetables and rice, arroz a banda (with shellfish, so that's not for you), arroz negro, and many others, and several ones don't have fish.
And if you don't find them interesting, in a specialized place you can always ask for a fish only paella, explaining your allergy to make sure they don't put the fish and the shellfish by error, and then separate them or that they don't use possible mixes fish/shellfish they may have prepared to go faster, just separating the shellfish.
Anyway, I wouldn't try this before trying the original one. maybe you'll like it more.
Ask for "una paella
sólo de pescado" and add "soy alérgica al marisco".