Can you return the friends?

Is it an ideal pairing? Probably not. Will it work? imho, well enough for where you are, your size, etc. Your life will be much easier with a floaty board and a wing that will lift you easily. The board should have plenty of volume and the foil wing enough area so that you come out of the water at a low enough speed that you don't need a lot of power and are comfortable.
There is no magic formula in terms of fuselage length for a given board size. The way a foil behaves depends on a bunch of things, just one of them being fuselage length. And, for a given length, how the foil behaves will also depend up your front wing chord, straps/mast base locations, stab size, stab angle, stab shape even the projection of the wing in front of the foil (ie two 115 fuselages can be different from one another such as the original SB 115 and the IQ 115). Shorter fuses tend to be less pitch stable. I say tend because I had an AFS 95 (RIP) that was something like 88, that was a stable as some 105 fuses because of the wing chord and stab set up. Generally, fuses from the low 90s to 115 work well for windfoiling.
As for balancing your gear, there are lots of tuning options to get things sorted - again, front strap location, mast base, stab angle and boom height all make a difference and in some cases a considerable difference. I'd reach out to both the shop and Severne and get a general range of recommended settings for the distance from the front tuttle screw to the mast base and start from there. I'm guessing it'll be around 102-108cm for a board like yours. I'm also sure that someone here will chime in, too.
Your shop is entirely correct. At your size and skill level, a smaller foil is going to make everything harder. Later, when you have better skills and have your jibes workable you can move to smaller but doing it too soon just makes everything harder than it needs to be. When good mid-aspect 1300-1500 wings became available a few years back was when so many people really started to have fun recreational foiling. Many graduated to smaller, faster wings as their skills improved, others have stayed solidly with those kinds of wings.
I would suggest that your board is a bit aggressive for your skill level if you use the back straps. I'd suggest starting without them. They are going to place your feet too far outboard on a board that has a fairly wide tail. Again, as your skills improve and you may want to try them especially for going upwind but at this point, they'll be more trouble than they are worth.
Not me but someone who's just been foiling a few months and just upgraded his gear to a set up similar to yours. His local shop is really good and could have easily sold him a racier foil but instead on something similar to what you have.