Yo no quiero salchichas hoy; prefiero un filete pequeño con ensalada.

That’s right: Spanish often drops subject pronouns because the verb ending already shows who is doing the action.

• quiero = I want
• prefiero = I prefer

So Yo no quiero salchichas hoy and No quiero salchichas hoy are both correct.

Including yo often adds:

• emphasis
• contrast
• clarity

Here, yo can sound a bit like as for me or I, personally.

In Spanish, no normally goes directly before the conjugated verb.

So:

• No quiero = I do not want
• No prefiero = I do not prefer if needed in another sentence

This is the standard word order for simple negation.

A useful pattern:

• No + conjugated verb + rest of sentence

Examples:

• No como carne.
• No tengo hambre.
• No quiero salchichas hoy.

Because the speaker is talking about sausages in a general, non-specific way.

• No quiero salchichas hoy = I don’t want sausages today, in general
• No quiero las salchichas = I don’t want the sausages, meaning specific sausages
• No quiero unas salchichas = I don’t want some sausages / a portion of sausages

Spanish often leaves out the article when talking about food or things in a broad, unspecific way.

Both verbs are stem-changing verbs in the present tense.

• querer changes from e → ie
• preferir also changes from e → ie

So:

• querer → quiero
• preferir → prefiero

This happens in many present-tense forms, though not all of them.

For example:

querer

• yo quiero
• tú quieres
• él/ella quiere
• nosotros queremos
• vosotros queréis
• ellos quieren

preferir

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So quiero and prefiero are forms you simply need to learn as part of the verb pattern.

Not necessarily.

If you only say what you prefer, no a is needed:

• Prefiero un filete pequeño.

If you compare two things explicitly, Spanish often uses a:

• Prefiero un filete pequeño a las salchichas.

So in your sentence, prefiero un filete pequeño con ensalada is complete on its own because the alternative is already understood from the first clause.

Because in Spanish, descriptive adjectives usually come after the noun.

• un filete pequeño = a small steak

This is the most neutral and natural order.

If you put pequeño before the noun, un pequeño filete, it can sound more literary, emotional, or subjective, and sometimes it suggests something like a rather small steak or gives extra stylistic emphasis.

For everyday ordering or describing food, un filete pequeño is the normal choice.

Because con ensalada is a general way to say the dish comes with salad.

Spanish often omits the article with foods and accompaniments when speaking generally:

• con ensalada
• con patatas
• sin azúcar
• con arroz

But you can add an article if you mean something more specific:

• con una ensalada = with a salad, meaning one salad portion
• con la ensalada = with the salad, meaning a specific salad already known

In a restaurant-style sentence, con ensalada sounds very natural.

Yes. Adverbs like hoy are fairly flexible in Spanish.

These are all possible:

• No quiero salchichas hoy.
• Hoy no quiero salchichas.
• Yo hoy no quiero salchichas.

The difference is mainly emphasis:

• No quiero salchichas hoy puts today later in the sentence
• Hoy no quiero salchichas emphasizes today more strongly

Your version is completely natural.

Yes. The semicolon links two closely related complete clauses:

• Yo no quiero salchichas hoy
• prefiero un filete pequeño con ensalada

It shows a close connection between the two ideas.

You could also use:

• a full stop: Yo no quiero salchichas hoy. Prefiero un filete pequeño con ensalada.
• pero: Yo no quiero salchichas hoy, pero prefiero un filete pequeño con ensalada.

The semicolon is a neat written way to connect them without adding another word.

In Spain:

• salchicha usually means a sausage-type item, often similar to a frankfurter or processed sausage, depending on context
• filete usually means a slice or cut of meat, often something like a steak or fillet

So un filete pequeño in Spain would usually be understood as a small piece of meat, often the kind you might order as a main dish.

Exact meaning can depend on the menu, but both words are very normal in Peninsular Spanish.