According to the article on this site it says that Bottle-brush is not terribly picky about soil type. So a generic slow release granule should be sufficient. Note: more nitrogen generally yields more leaves and green, more phosphate and sulfur will yield more flowers.
Fertilize bottlebrush shrubs for the first time in their second spring. A 2-inch layer of compost over the root zone makes an excellent fertilizer for bottlebrush. Pull back the mulch before spreading the compost. If you prefer to use a chemical fertilizer, follow the instructions on the label.
Read more at Gardening Know How: Growing Bottlebrush Plants – Learn About Callistemon Bottlebrush Care https://www.gardeningknowhow.com/ornamental/shrubs/bottlebrush/growing-bottlebrush-plants.htm