The biggest thing is knowing your dog's temperment and judging for yourself when they are ready. There is no magical age. Sadie was "uncarted" about 7 or 8 months when we starting giving her the house, Cincy was over a year.

Once you think your dogs are ready, do it gradually. Leave them out when you take the garbage out, or take a short walk around the block, maybe even just stand outside the door if you think that is all they can handle without tearing things up. Go back in and praise them for being good. Slowly increase the time you are gone. Never punish them for being bad, they will associate your coming home as a bad thing.

Also, if you think the the bulldog will hurt the chi don't take a chance! Give your chi a place to hid where your bulldog can't get to him, or keep them in different room, even if they are not crated.