Hi DL. Six hours of sun should be fine for both Mel's Heritage and Mary Rose. You may have some disease issues, particularly on Mary Rose, when the sun is lower, but it is sufficiently intense in Gilroy for six hours to be sufficient during the growing season. The "ideal" would be morning sun with afternoon shade because afternoon sun is hotter, but you have to play the hand you're dealt. Remember, when "roses need full sun" was written, it was very likely written in Britain where the sun and heat are MUCH less than they are here on the West Coast. The hotter the sun, the more washed out your colors will be and the shorter each flower's life will be, but you can utilize what you have and succeed. Good luck and have fun with them!

I'm in Sonoma, my temperatures are probably similar to San Jose, but not as hot as Napa or Gilroy. In general, compared to other roses, David Austin's roses are water hogs in our climate; especially when the flowers are opening. In hot summer temps the flowers will shrivel and melt if not given enough water. They also require steady water after the flowers are done to coax the next buds to develop or the flower size will shrink in half. You will notice a difference when the roots get deep enough to reach the underground water table. For heavy flower production, they also like to be fed, a lot. Once your plants get about knee height or taller any rose fertilizer will do until you find your favorite brand, but small first year bands or gallons from heirloom will get fertilizer burn if you're not careful. If you must feed small plants, go organic with fish emulsion or use liquid fertilizer with NPK in the single digits (follow the orchid growers' mantra: feed weakly weekly). Because I have gophers, I generally pot bands up to #1 gallons and then #5 gallons their first year before putting them in the ground in wire baskets. After your Austins get established and grow enormous, look into summer pruning to get more flowers.

I am not in your zone, but I have 41 Heritage and 8 Mary roses. Some of them are in partial shade, and they are doing good.

There is a tremendous difference between the sun and heat intensities between zones 6a and 9a. "Partial sun" in 9a, particularly here in Central California, could well be the equivalent of "full sun" in a lower zone almost anywhere else. I am also in 9a with a strong coastal influence, and I am finding almost no rose looks good with more than six hours of sun. The foliage is actually damaged by the severity of the UV intensity and just looks "crappy". It isn't from any spray damage as NOTHING is sprayed other than hose water to wash off dust/debris and aphids, mites, etc., and that is only done later in the afternoons when the sun has moved lower so the UV is reduced.