Ao pequeno-almoço, como iogurte com fruta.

Ao is the contraction of a + o (to + the). In this sentence it means roughly “at/during/for,” so ao pequeno-almoço means “at/for breakfast.” This is the most idiomatic way in European Portuguese to say what you have for a given meal:

• Ao pequeno-almoço, como iogurte…
• Ao almoço, bebo água.
• Ao jantar, como sopa.

It’s optional. The initial time phrase Ao pequeno-almoço has been fronted for emphasis, and a comma is often used after such fronted adverbials. You could also write it without the comma:

• Ao pequeno-almoço, como iogurte…
• Ao pequeno-almoço como iogurte…

Yes. Word order is flexible:

• Como iogurte com fruta ao pequeno-almoço. Fronting the time phrase (as in the original) just foregrounds the time context.

Portuguese is a “null-subject” language: the verb ending -o already shows the subject is “I.” Eu can be added for emphasis or contrast:

• Como iogurte… = I eat yogurt…
• Eu como iogurte… = I (as opposed to someone else) eat yogurt…

In Portuguese, the simple present often expresses habits. So the most natural reading is habitual: “At breakfast, I (usually) eat yogurt with fruit.” To emphasize habit, you can say:

• Costumo comer iogurte com fruta ao pequeno-almoço. To say you’re doing it right now:
• Estou a comer iogurte com fruta ao pequeno-almoço.

Both are possible but not identical in feel.

• Ao pequeno-almoço focuses on what you have for that meal (choice of food): “for breakfast.”
• No pequeno-almoço means “at the breakfast (event/time),” often used for things that happen during that meal. Examples:
• Ao pequeno-almoço, como iogurte. (What I have for breakfast.)
• No pequeno-almoço, falámos dos planos. (At breakfast, we talked about plans.)

When talking about foods in a general or mass sense, Portuguese often omits the article:

• Como iogurte = I eat yogurt (in general / some yogurt). If you mean one serving or one pot, use the indefinite article:
• Como um iogurte (commonly “a yogurt,” i.e., a pot). If referring to specific, previously mentioned items, use the definite article:
• Como o iogurte e a fruta que comprei.

Not quite.

• iogurte com fruta suggests yogurt that contains fruit pieces/pulp.
• iogurte de fruta is “fruit-flavoured yogurt” (often without real chunks). More natural is to name the fruit:
  • iogurte de morango, iogurte de pêssego, etc.

Approximate IPA: /aw pɨˈkenu aɫˈmosu, ˈkomu juˈɣuɾtɨ kõ ˈfɾutɐ/ Tips:

• Ao = like English “ow.”
• pequeno = pɨ-KEH-noo (the first vowel is like a reduced “uh”).
• almoço = al-MOH-soo; ç sounds like “s.”
• como = KOH-moo.
• iogurte = yoo-GOOR-tɨ (final vowel often very reduced).
• com = nasal “kohng” (õ).
• fruta = FROO-tɐ (tapped r).

In Brazil, pequeno-almoço isn’t used. They say café da manhã. Also, the progressive form uses estar + gerúndio:

• Habit: No café da manhã, eu como iogurte com fruta.
• Right now: Estou comendo iogurte com fruta no café da manhã.

Use para o pequeno-almoço when something is intended for breakfast (purpose), not typically for what you consume:

• Comprei iogurtes para o pequeno-almoço. (I bought yogurts for breakfast.) For stating what you eat for that meal, ao pequeno-almoço is the idiomatic choice:
• Ao pequeno-almoço, como iogurte…