I just had a similar experience in reverse so I think it would be ok as long as you go about introducing them properly (there are many threads on this).
I just found out that it looks like my 3-4 wk old silkie and 2 mo old EE are going to get along just fine.
After they ate this morning, the EE kept running up to the silkie and the silkie would run away and the EE would give chase. At first I thought she was trying to get at her, peck at her, but it seems like she misses the company of another chicken! Whenever the silkie stopped running, the EE would stop too! Wouldn't peck her, just hung out to see what the silkie did next, and then followed suit. She kept following her around.
I have to adjust their temp housing to make it more suitable for the teenier chick, but they move in full-time tomorrow.
For older birds like yours I definitely recommend the "slip the chicken onto the roost in the middle of the night and distract them with treats in the morning"" approach.