Is this rabbit wild? If so, I HIGHLY advise you to return it to where you found it, as they are wild animals and do not belong in captivity, especially when taken from a perfectly safe and natural situation. Additionally, cottontails and European rabbits are different species and cannot breed. Moreover, wild rabbits can carry many parasites and diseases that could harm or kill your domestics. If the rabbit is domestic, don't release it, but I still wouldn't house it with your current rabbits for fear of disease.