For a rough guideline, you can start giving her porridge after a month of introducing cereal. For fish/meat, some mummies introduce when 8-10mths old but some mummies introduce as early as 4mths old.
I started Iggy on porridge with fish/meat only when he was 7mths+, which is 1 month after cereal. When I 1st introduce him to porridge, I blend the porridge before giving him, so that the texture will be similar to cereal.
Maybe you can try mixing the banana with cereal?