Mon plat préféré pour le petit-déjeuner préféré

Your suggestion is correct but it's easier to write it like that ("My favorite breakfast")

Mon platetit-déjeuner préféré est le gruau.

In general, "plat" is used as "dish" so same here, it's better to say "Mon petit-déjeuner préféré" but both are correct.

Généralement, je le mange avec ldes fruits et ledu yaourt.

Here, you're talking about fruits and yogurt in general (indefinite) so you use "des / du". "Les / le" would be correct in a sentence in which you specify which fruits and yogurt you are talking about (definite). For instance, "les fruits de mon jardin" (the fruits of my garden) and "le yaourt du marché" (the yogurt I bought at the market).

Si je ne mange pas de gruau, je mangerai des céréales.

We keep the present here as you're describing a habit. "If I don't eat porridge, I eat cereals".

Mones céréales préféré es sont "Avoine Croquante".

We use the plural here because cereal packagings have multiple cereals inside.