Bridge mode is basically two, dual, amps driving a load with one signal. To achieve this, one of the amps output voltage is reversed by 180 degrees compared to the other channel, and the load [network] is cabled between the two positive [red] posts.
Since the lowest specified Dual mode output load in the literature is 4 ohms per channel, in Bridge mode it would be the sum of the two loads, each at 4 ohms, hence 8 ohms! so yes, it is safe to run Bridge mode with 8 ohms. I would not try a speaker network of less than 8 ohms.
I am driving one speaker ..where should I connect the neg speaker wire?