If there is no surface water connection to another pond or creek, then I don't see any risk of invasive fish coming in via groundwater.
If the flow of water from your outlet will not create a continuous water connection to a creek, then you should be safe from fish moving upstream. However, they will move upstream from surprisingly little water flows!
How are you going to keep your pond excavation area dry enough to work if you are creating a groundwater pond? You may need to cut a sump and a drainage trench to convey water out of your worksite and far down the slope away from you.
If the geometry of that does not work, then you probably need to dig a sump and plan on running a trash pump while you are excavating the pond. Wet clay is NOT FUN when combined with heavy equipment.
Good luck on your new pond!
