Are you planning a wholesale or retail operation, or both? I would go wholesale. This way you would not need a storefront location. A warehouse would be fine. You would have a much larger market too; as most retail shops will not buy from other retail shops and certainly not at retail pricing.
Currently, many fish based stores are moving into the VA area. Make sure that by the time you are all set up, there is not an oversaturated market.
In terms of the rays, I think there is a large demand. Currently, there are no local distributers other than middle men wholesale shops (most out of NJ). Often their quality of rays are poor and don't ship well. Transhipping is one way to go, but Brazil is cracking down on transhipping rays and most transhippers have no guarentee on rays whether or not they arrive doa.
I do not think it is a wise venture to offer more commonly available fish. You have a good plan with rays and altums. Most stores carry decent discus and with DiscusHans in Bmore, you may be too close to offer a competitive product. I would focus my attention on filling in voids in the hobby where demand is measurable but supply is low. Rays, black arros, high grade flowerhorns, and wild caught less common cichlids.
As for offering a maintenance service, good luck. The metro area is saturated with them. Almost every store that has been around for 10+ years offers this service in some capacty, as do many hobbyists who have spent time working for their lfs... especially on tanks larger than 75. The store I manage has many contracts on large, custom tanks. Most are set-ups we installed, but not all. All are well over 100+ gallons.