First off, welcome aboard!
Anyway, you've discovered the frustrations of mixing paths and buildings... To be honest, there's really no good way to do this. You usually have to choose between which of 2 ways, both of which look bad, looks worse for your particular setting. Avoiding those 2 issues requires a lot of work but produces the best results.
The issue is that you can only deselect curbs on paths at ground level. If the path is even 1 inch above the ground, it will always have a curb. This curb is the "top" of the path as far as making a gridded path goes. So, if you use the building itself as the grid for the path, the path will be completely invisible because the top of the curb will be just below the top of the floor tiles. HOWEVER, the actual walking surface will be the curb's height below the floor, so peeps will appear to be wading ankle deep in the floor tiles. OTOH, you can build the gridded path first, then place the building over it and adjust its height so that the floor tiles are only 1 pixel above the path surface, so the peeps will appear to walk on the floor tiles. However, the bath curbs will stick up through the floor.
The ONLY way to avoid this dilemma is if the gridded path is at ground level and you then position the building so that its floor tiles are only 1 pixel above the path surface. With the path on the ground surface, you can turn off curbs so they don't stick up through the building's floor. However, this completely prevents having a raised sidewalk between the building and the street outside.
With that said, you have 2 ways to go about this. First, if you don't need a raised sidewalk outside the building, look at the building and determine what size of gridded path areas you need for the main floor space. Do NOT have this gridded ares go right up to the shops. You'll have to connect the shops to the gridded plaza individually and need some space between the shops and plaza to do that. Then build this gridded plaza on the ground with curbs disabled, at some distance from the main path so you can connect the gridded plaza to the main path with a separate piece, where the building's door will be, to keep peeps from walking through the walls everywhere it fronts the main path. Then place the buliding, use advanced move to rotate it parallel with the gridded path, lower it until the the floor tiles are 1 pixel above the path surface, and move the building in the horizontal plane so that it covers the necessary part of the plaza in the right place. Then connect the shops to the plaza. Done.
The 2nd way is if you need a raised sidewalk between the main path and the building. The easiest way here is if you've decided that peeps wading in the floor is better than curbs sticking up through the floor. In this case, place the building first and adjust its height to get the proper sidewalk height and distance off the main path. Then make a gridded plaza inside the building using the build's floor itself as the reference grid. Then connect this plaza to both the main path and all the shops. If you don't want wading and don't mind protruding curbs, then you set the vertical grid height to minimum, raise the path grid to sidewalk height above the main path, then build the gridded plaza at that level, then place the building and adjust its height so the floor tiles are only 1 pixel above the path surface.
There is also a 3rd way to have a raised sidewalk and have neither protruding curbs nor wading peeps inside the building. This requires raising the terrain to sidewalk level in the area inside the building, so paths there can avoid having curbs. First, place the sidewalk beside the main path just hair lower than you really want it. Then raise the terrain on the other side of the sidewalk flush with its top, over the necessary area Build a gridded plaza of the required size, with no curbs, atop this raised ground. Then place the building there, rotate as necessary, and adjust its height so the floor tiles are 1 pixel above the plaza. Then connect the plaza to the main path and each shop.