I'm new to modeling myself so I can't be of much help but if you'll watch the cartoon dog tutorial throughout you'll see that the author had the same booleans union problem when attatching the dog's rear leg. What he did was use the split polygons tool to cut out an opening in the dog's body similar in size and shape to the leg portion that is to be attatched. He then simply merged vertices to complete the union. Basically, the same thing as a boolean union only you do it manually and without the boolean operation but it still ends in the same desired effect.

From what I gather, boolean operations are nice as they often save you lots of time, but by nature don't always work as you intend them to. I'd suggest learning to do it the long way (cutting a hole with split poly tool then attatching via merge vertices) first so that you won't be handicapped when your boolean union fails again. At least that's what I've been doing. I started by using simple primitives to practice the operation on and have since moved on to more complex poly shapes.

But like I said, I'm a newbie at this as well so take what I say with a grain of salt. I don't want to inadvertantly give anyone bad/wrong advice as I'm nowhere near qualified to be giving advice at all. It's just that no one seems to have replied to your post and I thought I'd offer what little help I could. Good luck and I hope you can find an answer to your problems soon.