Adding downthrust will not prevent a model from climbing when you add power if it's trimmed for level flight at less than full throttle. Downthrust just changes how much the nose will move when you change power. Even a Pattern-type airplane that's a neutral as possible will climb when you add throttle if it's trimmed to fly at less than full throttle.

The only way to prevent a climb when you add throttle is to trim the airplane for level flight at full throttle. Just be prepared for a good nose drop when you reduce throttle. This also means that the final approach glide may require significant 'up' elevator to keep it at the proper glide attitude.