I do not think that there is a problem with having both. I have both on my '98 and they do not interfer with each other. I do not think that there is that much difference between the placement on a '99. Ask the company that you buy the parts from.
Unless your car is lowered, CC plates shouln't be needed...and then it should only need them if the car is lowered more than say 1.25"...
And you probably won't really notice a difference by adding the STB...they stopped putting them on Mustangs in 98 for cost reasons, but if they were needed they would have kept them on there...All it might be good for is to look good under the hood IMO...