There are a number of obvious preferences before it should go into a tank: the wood should be dead, clear of bacteria and larger organisms, and depending on your preferences, devoid of significant tannins.
I think you're right to be concerned over smelly and slimy wood. Are those areas soft?
If it's too big to boil, then an option is first scrub it down (again), perhaps with a stiff bristle or a wire brush, then a 24-48 hour bleach bath.
The brushing should only be hard enough to remove loose debris and any obvious areas that might harbor infestations of wood rot.
The bleach can be diluted. Afterwards, I'd give it a few days in clean water, changing the water a couple times.
Boiling will help remove tannins, but if you are fine with tea-colored water (personally, I like the look), or if it has little tannin, you don't have a need to worry about that.
Rotting wood or wood with obvious wild life should probably never go into most tanks.