Although there may not be regulations against it, most agencies should highly caution you when putting an older adopted child in with another child…at least for a while. Depending on your adopted child’s history, it may not be a desireable situation to have him with your biological child. For instance, an institutionalized child may exhibit inappropriate behaviors that were never regulated in his orphanage such as mutual masturbation. When children don’t get enough attention, they will often turn to these behaviors to feel loved…and it feels good. If your child came from an abusive household, he may be abusive to your bio child as well.
If you do proceed, I would advise you to be aware of all the possibilities and prepare your bio child for what to do if your new son exhibits inappropriate behaviors behind closed doors. You may also have to lay some ground rules about when (if ever) the door can be closed, what is expected during “sleep” hours etc. Do not assume your adopted child knows what these seemingly basic acceptable behaviors are.