I think an AI can be bribed to attack another that has a higher military ranking. IIRC, I have managd that several times, including a current game (more below). I do agree, though, that if an AI has a high mil ranking and another has a low one, that it seems impossible.
In a tight game, it can be vital to keep track of which AIs have treaties and trade routes with each other. For example, even trade routes produce income for them and a war declaration stops that, just as a research and eco treaty being lost stops that source of resources. Additionally, the freighters can end up coming to the player, just as long as you keep out of wars.
I have found getting and keeping all the other AIs into early wars amongst each other to be a key tactic to winning on Suicidal. Being in other wars often makes them "too busy" to declare war against the player.
The game (DA) I am currently playing is one in which I waited too long. The Drengin got too big earlier than I realized, decided their colony rush was over, and declared war on me. I got several others to declare war on them (despite lower mil rankings), but it has become a hard (but fun) game. I look to have them on the ropes now, but we have traded invasions of border worlds for a few score turns now. They sent multiple fleets immediately at my mining bases. I had to cede the purple, yellow and even the blue ones, but have been doggedly defending my one green one and my two red ones. They destroyed one red one after killing my defending fleets, but I have retaken it. Meanwhile, I have been nibbling away with invasions on their other flanks.
Also, in the meanwhile, one of the other AIs that has kept out of the war has grown and quietly began gobbling Neutrals. I had to bribe them to peace, as the Neutrals are important to my economy.
A fun game!
