I don't have a TM... just an EM... but, I would see how they two dogs interact - are they happy to see each other, just disinterested or do they avoid contact?
We adopted a 1 yr old Bulldox/boxer to be a sibling for our 3 yr old Dane/Lab/x a while back (we don't have either, anymore... it was a while ago), and their first interactions were mostly disinterest (especially on the new dog looking at our resident dog). They had some good play time, but never did hit it off as comfortable siblings in the long run.
I would look for a spark of happiness between the two - that the TM is interested in knowing more about your resident dog, even if he's shy about contact... interest in a soft, polite manner would be a very positive reaction in my book.
I would make sure you stand up for your resident dog, ignore the TM more in the beginning (so he can assimilate by observation), and step in anytime you see things getting 'out of hand' (to YOUR comfort level). Make sure they both respect each other's space, and enforce the house rules fairly.
Also, know that it will take a few months (at least) for the TM to show his 'true colors' once in your house. Once he's comfortable, he'll open up and let you know who he really is... hopefully that is a Very Good Thing. If it's not... which is what happened with us... make sure you are ok with finding him a home where he's more comfortable.
That's just my very small experience talking... hopefully you'll get a lot more (and better) info from the great group of experts here on the forum.
Oh.. and... WELCOME to the Forum!