Yes, roaming will be fine between R500 and R600 — the only gotcha is when you are in an area with overlapping coverage between the R500 and R600, you won't have much control over whether a device wants to pick the R500 or R600 — most client roaming algorithms do not choose one AP over another based off much more than signal strength — they will not take into account whether an AP is 2 or 3 spatial stream.
FWIW, in a sparse suburban area, I've had good luck with a 7982 indoors transmitting ~5 ft away from a window covering a very large backyard area with nearly full signal, primarily on 2.4GHz. So the coverage will likely be sufficient, based off what you've said.
I'm sure you're aware, but Ruckus also makes purpose-built outdoor AP's, the T300 series for AC, and the 7782 series for 802.11n. They are more suited to the task of covering a larger outdoor area. But if it's just a small garden, an indoor AP in the garage might be sufficient.
