You have two things working against you - breed temperaments and wide age spread.
Wyandotte temperament is "edgier" than Orpington docility. Add that to being the younger and smaller and less experienced, and you don't stand much of a chance for smooth integration.
Adding chicks during the first couple weeks is relatively without conflict, but your Wyandottes, besides being naturally more aggressive, are six times larger. You aren't going to be able to let them merge for a while.
But keep them all together like you have been. That's half the battle - letting them be aware the other group exists and they are one flock.
What I recommend, when the new ones are a couple weeks older, is to utilize a panic room for the smaller chicks. Just rig a temporary safe enclosure with openings that only the small chicks are able to fit through. This creates a safe refuge for the more timid chicks, and they can retreat to it whenever things get too rough. It will give them time and space to grow their self confidence without having it beat out of them every day, which is what the older chicks are likely to do, given their immaturity and inclination to bully for the sport of it.
A lot of conflict is over food and water, so place these essentials in the panic room to make sure the Orpingtons continue to be well nourished and able to withstand the stress of integration. By the time the Wyandottes reach six months, they should have settled down a lot.
