I don't have any experiance with the 609 on buckets over 24". I'm in a silt/clay mix with little to no rock. Digs well if there is some moisture. When dry it is quite hard and difficult to penetrate.
Bradco makes buckets for the 500 and 600 series up to 38" wide.
Bucket size
Struck cap. ft.³
Heaped cap. ft.³
Weight lbs
Number of teeth*
12''
1.43
1.83
140
3
16''
1.98
2.56
154
4
18''
2.25
2.98
162
4
20''
2.52
3.40
168
4
24''
3.07
4.32
185
4
34''
4.23
5.80
238
7
36''
4.48
6.14
257
7
38''
4.73
6.48
269
7
* buckets available without teeth
It is not the weight of the material in the bucket, it is whether you have enough power to get the bucket teeth to penatrate the material. Wet soil is about 2700 lbs per loose yard. So a heaped 36 bucket carries about 614 lbs of material, the 18" bucket about 298 lbs.
7 teeth vs the 4 teeth on your 18" will take more force. If you have easy digging conditions in your area, I would think you should be ok with a 36" and you'll be moving near twice the material per cycle. But that only applies if it will penatrate and scoop near the same speed as the 18". Remember that 36" is probably more that half the width of your loader bucket! You might also call Bradco for their opinion.
I have only seen a couple wide bucket on mini-excavators in my area. They were smooth edge and being used for grading and mucking ditches.
In a 2006 Bradco catalog, I did find the adapter below that allowed you to use C&P #26, and #27 style buckets. (Same as WAIN-ROY). But at $885 I wonder how many they actually sold?