A colazione mangio yogurt con cereali e miele.

A colazione is an idiomatic expression meaning at breakfast or for breakfast.

The preposition a is commonly used with meals:

• a colazione = at breakfast
• a pranzo = at lunch
• a cena = at dinner

So A colazione mangio... means At breakfast, I eat... / For breakfast, I eat...

Yes. Per colazione is also very common.

The difference is slight:

• A colazione mangio yogurt... = At breakfast, I eat yogurt...
• Per colazione mangio yogurt... = For breakfast, I eat yogurt...

In many everyday situations, both work well. A colazione can sound a little more like setting the scene; per colazione can sound a little more like specifying the meal choice.

Italian usually leaves out subject pronouns when they are already clear from the verb ending.

• mangio = I eat
• the ending -o already tells you it is I

So:

• Mangio yogurt = I eat yogurt

You only add io when you want emphasis, contrast, or clarity:

• Io mangio yogurt, lui mangia biscotti.
  I eat yogurt, he eats cookies.

Because Italian often leaves out the article when talking about food in a general or indefinite way, especially in simple statements about habits.

So:

• mangio yogurt = I eat yogurt
• mangio cereali = I eat cereal / cereals
• mangio miele would be less common because honey is usually something you have with something else, but the same idea applies

If you add an article, the meaning can become more specific:

• Mangio lo yogurt = I eat the yogurt / sometimes I eat yogurt with more focus on the item as a category
• Mangio i cereali = I eat the cereal

In beginner terms, the version without articles here is very natural for a habitual breakfast sentence.

Con means with.

So:

• yogurt con cereali e miele = yogurt with cereal and honey

Here con connects yogurt to both cereali and miele. In other words, the cereal and honey go with the yogurt.

Yes. In this context, cereali commonly means breakfast cereal.

Literally, cereali can also mean grains or cereals more generally, but in a breakfast sentence like this, an Italian speaker will normally understand it as breakfast cereal.

So:

• yogurt con cereali = yogurt with cereal

Yes. That is also correct.

Italian word order is more flexible than English word order. These are all natural:

• A colazione mangio yogurt con cereali e miele.
• Mangio yogurt con cereali e miele a colazione.

The difference is mainly emphasis:

• starting with A colazione highlights breakfast
• putting it at the end sounds a bit more neutral

In modern standard Italian, colazione on its own normally means breakfast.

Examples:

• Fare colazione = to have breakfast
• A colazione bevo il caffè. = At breakfast I drink coffee.

You may occasionally see expressions like colazione di lavoro, which refers to a business lunch, but in an everyday sentence like this, colazione clearly means breakfast.

Yogurt is a loanword, but it is completely normal in Italian.

It usually stays the same in the singular:

• lo yogurt

The plural is often also unchanged in everyday usage:

• gli yogurt

But in your sentence, that does not matter, because there is no article and the meaning is just the food in general:

• mangio yogurt

Yes, absolutely. It sounds natural and everyday.

It is a normal way to describe a breakfast habit:

• A colazione mangio yogurt con cereali e miele.

You could also hear close alternatives such as:

• Per colazione mangio yogurt con cereali e miele.
• Di solito, a colazione mangio yogurt con cereali e miele.
  Usually, for breakfast I eat yogurt with cereal and honey.

So the original sentence is perfectly good, natural Italian.