Adding fish during a planted cycle?

If you have ammonia or nitrite in the water and you add a fish, you will probably kill the fish.

If you have ammonia or nitrite in the water and want to add a fish, replace all the water so there is no ammonia or nitrite and then add the fish. Make sure the new water is free of chlorine/ chloramine before it is added to the tank.
Keep the feeding down to once every second day and do a 75% water change each day. Monitor the ammonia, nitrite & nitrate levels, and when the ammonia & nitrite stay on 0 and the nitrates start to go up, the tank is cycled and you can feed more often.

 

Another thing...you mention plants, so if the plants are growing, they should easily deal with ammonia from one Betta.