Here is what I would try. Let the longnose sit a day or two as long as you moved it. Then I would wait until just before the tank lights go out move the decorations or a few live rock pieces around put the Puffer in and turn out the lights. I have found the old trick of turning out the lights for a few hours or overnight sometimes makes a world of difference. Also I would feed the tank just before slipping the Puffer back in. Now the only thing unanswered in your post was who is the bigger fish I assume it is the new one as Valentini's (at least mine) never have been bigger then 3 inches and frankly I have only had one that was a bully and that was toward a second one I tried to add after the fact. Now also I do not remember if you ever have stated the size of the tank because a small tank could be a problem for these two to get enough of their own space. Lastly I have seen Puffers and other fish that give each other heck for a day or two then settle down and live well. As long as they are not acting like they want to tear each other apart I think a little pschological work as outlined above may make their transition easier on both.
I also must admit that I am not sure what they are calling a longnose as I have not heard that term around here. There are Dogfaces that have long snouts and their are other flatter Valentini types that could be considered a long nose, and if these two are the same shape and size frankly this is going to be a problem no matter what you do. If their shapes are the different types then patience should help you to make it work. I am very interested if this helps you post me back here or thru my homepage to my club page or even my e-mail where I can get back to you a bit faster.
