Hi Saranya
There is no harm in eating your breakfast and doing pooja .Those days people said you cannot eat before pooja so that you don't have to take bathroom breaks .I do all my poojas after eating a heavy breakfast and keep the satham and paruppu for maha neivedyam . no harm in in it .but My MIL usually drinks only liquid diets before pooja .but I cannot do that since I cannot have tea/coffee and I never avoid my breakfast .
SO please you can eat a good breakfast and do pooja without the growling stomach or being tired