SRAM and Shimano dont mesh, dont let anyone tell you otherwise

So, Im in a tiff over this. When i was in Vancouver, I was talking to the owner of a bike store. He had a booth at the Expo and I was admiring one of the Argon18 bikes. We got to chatting about SRAM and how I had to switch cassettes so I could use my road bike wheel (shimano) on my TT bike (SRAM) for training. He told me the two were interchangeable. Um, not on my bike they arent. I dont know what stuff he is talking about but when I went out for my two hour ride last night, I put the wheel with the SRAM cassette onto my Shimano equipped road bike. It worked fine when I spun it around WITHOUT LOAD. My ride started downhill with some 39-26 gearing for a couple short steep climbs out of the 'hood but when I got down to the meat of my ride, the gears were grinding, slipping, crunching, etc. All things you dont want to hear when you have to climb a bit and put a bit of effort on the flats. My massive quad(s?) must have too much power for the axel and it torques so much that the gears just wont work, even with small adjustments so I just rode home after 30 minutes and finished riding on the trainer with the proper wheel for the proper stuff. I hate riding the trainer.