I don't want to rain on your parade, but a bed full of low growing roses means you can't plant anything else with them. Some ground cover plants will be okay, won't affect the roses, but it makes it more difficult to feed and mulch the roses as time goes by as the ground cover spreads, unless you control it so it doesn't get closer than 10 inches all round each rose bush.

8 x 3 isn't very large, so if you used something like Korresia, a yellow flowered fragrant rose that only reaches 3 feet, there's really only room for two of them - roses need good airflow around them to discourage fungal infections, so shouldn't be planted closely because you don't want the topgrowth mingling together.