If you're going to mix them, do it outside in a neutral area if you can - don't put them all indoors in a confined space straightway, otherwise you'll be asking for trouble. Try and use a field which isn't too muddy (impossible here at the moment!) because the small ones will get chased by the older ones and may slip and get injured.
The younger ones will get bullied to start with, but eventually they should all get on okay. You may have problems ensuring the younger ones get sufficient food, though, so that's something of which to be aware.