grammatical gender and number.
smaczny - describes masculine noun- singular
smaczna- describes feminine noun-singular
smaczne- describes neuter nouns- singular, and most plural nouns
(śniadanie is singular neuter)
smaczni- this is not the best word- it describes plural, masculine personal nouns (male people).
Pyszny/a/e is delicious. You can also say przepyszny/a/e for something very delicious.
"Delicious" is accepted. I will add that it's not that much stronger a word than "tasty" to my ear, but English does vary widely.
No. "Śniadanie" is neuter in gender, so you must use the neuter form "smaczne" to match.
You can use „śniadania są” or „śniadanie jest”. The word „śniedanie” doesn't exist.