Good luck finding a space! That downtown location does sound awfully expensive, and synergy or no being a little (just a little) out of the way isn't too bad of a problem. From my experience as a customer, most shops are not in high-traffic downtown areas. The two I can think of that were are either out of business or relocated to cheaper locales...
I do want to say what, in my experience, separates a good scooter shop from a bad one. Location, mechanic quality, selection of bikes and accessories - of course all of these matter. But before that you have to come at it with the right mentality, and I think you are - you have to start as a scooterist. Scooter sellers fall into 3 major categories, as I see it: 1) motorcycle shops looking to fill out their product selection; 2) opportunists selling cheap scooters during gas price peaks; and 3) scooter shops run by people who just like scooters.
The first two may succeed, somewhat, but they isolate their customer base. Motorcycle shops treat scooter buyers as second-rate customers - if you're on commission, are you going to pay attention to the $3,000 scooter buyer or the $15,000 Ducati buyer? #2 types tend to be all volume, no support, and garner no sense of community to build a solid customer base with. The last, #3 shops, are the best. Their love of scooters is infectious and makes the sales pitch, customer service, and after-sales experience all the better. Communities will support them - I know a lot of scooterists that travel to suburban dealerships rather than go to shops in-town because they get better service. Some don't bring enough business savvy to their love of scooters and go out of business too quickly.
It sounds to me like you'll be in category #3. Even if I don't have a ton of confidence in your choice of brand you at least are coming at this from the standpoint of a scooter rider - albeit one that apparently knows how to actually run a business. I wish you the best.