Jo només vaig demanar una sopa i una amanida.

Not usually. Catalan often drops subject pronouns because the verb already shows the person.

So Només vaig demanar una sopa i una amanida is a perfectly natural sentence.

Adding jo usually gives:

• emphasis
• contrast
• or a slightly more explicit tone

For example, Jo només vaig demanar... can suggest something like I only ordered..., maybe contrasting with what someone else did.

Vaig demanar is a past tense called the periphrastic past in Catalan.

It is formed with:

• a present-tense form of anar
• plus an infinitive

So:

• vaig demanar = I asked / I ordered

Even though vaig comes from anar meaning to go, in this structure it does not mean movement.
So vaig demanar does not mean I go to ask here. It is simply a normal way to say the past.

Yes, demanar often means to ask for, to request, or to order.

In a restaurant context, Catalan commonly uses demanar where English often uses order:

• demanar una sopa = to order / ask for a soup

This is very normal Catalan.
Also, beware of ordenar: in Catalan that usually means to put in order / arrange, not to order food.

Només means only / just.

In this sentence, putting it before the verb phrase gives the idea that the speaker only did that or did not order more than that:

• Jo només vaig demanar una sopa i una amanida

In practice, this often corresponds to English I only ordered a soup and a salad.

Word order can shift a little depending on what you want to emphasize. For example:

• Jo vaig demanar només una sopa i una amanida

This puts the focus more directly on the fact that those were the only items ordered.

Because they are two separate singular nouns, and Catalan normally repeats the article:

• una sopa i una amanida

This is the most natural standard phrasing if you mean one soup and one salad.

Leaving out the second article, as in una sopa i amanida, sounds less natural in ordinary standard Catalan.

Because both sopa and amanida are feminine nouns.

So the indefinite article must agree with them:

• una sopa
• una amanida

A useful pattern is that nouns ending in -a are often feminine, though not always.
It is still best to learn each noun together with its article:

• la sopa
• l’amanida

Sometimes you may hear article-less food nouns in certain styles, such as:

• menu language
• headlines
• very compressed speech

But in a normal full sentence, una sopa i una amanida is the safest and most natural choice.

So for a learner, this sentence is better:

• Vaig demanar una sopa i una amanida

Yes, but the meaning or style may change.

Here are the main contrasts:

• vaig demanar
  A completed past action: I ordered / asked for

• he demanat
  Present perfect: often used for a recent past, though usage varies by region

• demanava
  Imperfect: this would usually mean I was ordering, I used to order, or it would set background information, so it is not the same as this sentence

So if you want a simple completed event, vaig demanar is exactly the right form.