I did some free engineering for you... quick and dirty calculations and I think there is something wrong with your numbers. I assumed high bay industry (40 workers) and retail space (66 customers) in your metal building with NO WINDOWS and you will already need 20 tons in the production area and 10 tons in the retail space. Without visiting the site, windows measurements, counting workers, having actual orientations my numbers can be anywhere from low or high (see attached for calculations). But for sure, there is no way 9 tons will work for a 6000 SF & 15 foot ceiling metal framed building. I don't know how critical it is to maintain 60 F everyday... are you are okay with 74-78 F on a 91 F day.
To answer your other question, one perk of multiple units is redundancy and energy efficiency. A single 9 ton have several problems. (1) Most manufacturers only makes 8.5 tons, then skips the 9 ton with the next being 10 tons. (2) I wouldn't be surprised if 3 ductless mini-splits are cheaper than a packaged heat pump. (3) They are also trying to avoid structural engineers, by not putting units on the roof. Your 9 ton will probably need to be on the ground too unless you want to hire a SE.
It needs to be 60 constantly. 2 workers average in the 4500 sf production area.