Welcome and congrats to having cycled your tank first! The sky is the limit in regards to what tank mates to add. I have a female pearl gourami in with several schools of tetras in my 29gal. Schooling fish add color, activity and are interesting to watch, but single, non-schooling fish tend to have more of a personality IMO. You could do something like an angel, a gourami, a platy, etc.. as 'single' fish. My emerald corydoras are neat little buggers, but they are very skittish.
Make sure you still have a means to measure your ammonia, nitrites and nitrates after you add more fish. Having that test kit is essential.
What type of lighting do you have for your 40? Does the tank get natural light? Good lighting is important for plant growth. I've had zero luck attempting plants with standard fluorescent and LED tank hoods. Once I switched to lighting intended for plants, they took right off.