I am in the planning stages for the radiant system. I will also have an AC ducting system installed for the summer. In the winter I am worried about not having enough airflow in the house? Is this really a valid concern? Is there something that can be added or done to the AC ducting system to make sure there is airflow if needed.

Am I just worrying about something I should not be?

Yes, you should get away from the single mode air conditioning mindset and put in a heat recovery ventilator that will be designed to run/cycle year round. For summer cooling, the first step is to reduce/eliminate the cooling loads - exterior shading, tinted glass, etc. Then you can put a cooling coil on the supply air duct from the HRV to use for summer cooling and humidity control. Three legs of the stool: radiant temperature control, ventilation, and humidity control. You need ventilation year round for two purposes: one - primarily healthy indoor air quality, and two - humidity control if you are in a hot/humid summer design climate. If you have cold, dry winters, adding a small humidifier to the HRV's supply air duct will do it. If you really want to have the "full meal deal" consider radiant cooling - you already have the tubes in the structure anyway, and it means using a small water chiller (or heat pump, or other cool water source) to provide cool water to the radiant system, and colder water to the air side for de-humidification if required.