Don't worry about it too much. If anything it's the other way around - the old dog should be crated and the new dog allowed freedom initially. However, i'd just let them sort themselves out - and when you see that one is the top dog - don't worry about it - that's what is supposed to happen and better if it's sorted out quickly than not.
One thing that is probably a good idea is to thoroughly clean the whole house so the smell if your current dog is minimised - so they can both start on 'neutral' ground from the beginning. Most dog communication comes from smell.
Buy new dog beds for both of them. Make sure the new dog has it's own place and that both dogs know which bed is 'theirs' so at least they have somewhere to go if they want to get away.
If they start to fight and it doesn't look like they'll get along, try changing their diet to a food with less sugar (most dog food is very high in sugar (bulking agent)), more vegetables, more oils, more protein and less carbs... just to make them more content and less grumpy generally.
Also to make sure they know you are top dog when you feed one thing you can do is make them both sit on one side of the room, mix the food, place it down in the other corners and then wait, look at them and only let them go to eat at your command - one dog at a time - a different dog each day. Always calmly and almost silently as you do it all. The underdog to go first more often than the top dog (it's a signal to the topdog that you care for the underdog - that he is part of the family and not to be bullied).
Anyway good luck with it all i'm sure it'll be fine!