As with many things, DOSE, is the key concern. A little is great, a lot can do serious damage.
Manure, if fresh, is to "hot" for a tree in a flower pot. But composted manure (more than 6 months), is pretty good stuff. It also matters some which animal was the source of the manure. For example horse manure tends to be high in salt, not good for gardenia. Cow & goat manure is better. Chicken & most poultry manures are rather high in calcium, gardenia does not like high calcium in their soils.
Generally for bonsai we want slower, rather than more rapid growth. Nice steady, healthy but slower growth. So we feed very lightly. We don't want the trees to grow coarse with big leaves and long internodes. But we do feed enough to keep everything nice and healthy.
Coffee grounds, I've used them, too much is a "bad thing", again go light. You want less than 10% of your soil mix to be coffee grounds, I'd shoot for 5 % tops.
Gardenia want an acidic soil reaction. I would use a blend of about 25% to 40% peat moss, at least 50 % should be inert particle for drainage, like perlite or pumice. Sand is not particularly good for the inert component, its particles are too smooth, doesn't hold enough air or water. But in a pinch sand can work. The remainder you can put in some of your other ingredients, like the manure, coffee grounds, and or douglas fir bark - the seedling grade used for orchids is perfect. Key is to not overdo the "active organics" like manure and coffee grounds.
If you have a bonsai supply place available, I would use a blend of Akadama and pumice for my gardenias. Or Kanuma and pumice. That would be my choice.