Hi,

usually, introducing a juvenile ADF (about 1.5 cm SVL) to a group of adult ADFs doesn´t cause any real problems. The younger one might need more frequent feeding, so you would have to find a balance so that the new one can grow normally without overfeeding the others.
However, I wouldn´t introduce new frogs without quarantining them first anymore; there is always a risk of catching a possibly deadly disease (I´ve learned that the hard way)...