can't tell from here but I have had shrub type mini's lose all there leaves from overwatering.Once the process starts its grin and bare it.They came back.can't say if thats your problem.I pick yellow leaves once or twice a day on mine but they are all over.It only takes one or two to distract from the bush.

And they might just be older leaves that are finished with their work.

On your second question, it depends on the mini. Sorry if that seems like the response to a lot of questions, but minis can either stay compact like Si or grow into beasts like Incognito (currently hovering at 2'tall x 2' wide and it's early in the season). Mini-floras can get even larger, Conundrum is on it's way to 3' x 2'. So, without knowing which minis you are considering, I would say 3 minis in that bed. But there may be smaller ones that you can choose, and you can get in 4.

You might consider planting your minis on a 24" center pattern in your area. Be sure to check the pH of this new bed as roses prefer a 6.5-7 pH to do their best.
The yellow leaf problem could have a number of causes...old leaves, overwatering, or not the correct amount of fertilizer.
Hope this is helpful.
Meg