As far as I've noticed, all doffs have a chance to proc, independen of eachother. Let's say in case you have 3 very rare bfi doffs, each with 50% chance to proc, the problem is that all of them may proc, thus having 3 TSS procs, one for each. Thing is that even with 3 bfi doffs you may not have any proc at all every once in a while. I think the fix for this would be to cumulate the individual % of each doff into one proc chance. So in case of 3 very rare doff, you would get 50% x 3 = 150% chance to proc (which in this example you'd only need 2 of them to have a proc guarantee. This way it would be the equivalent (or almost the equivalent) of having a TSS ability and also we can avoid triggering 3 procs in the same time which gives the ultra shield resist ability. With only one TSS proc a ship would be much easier to destroy.
Hear! Sons of Kahless
Hear! Daughters too.
The blood of battle washes clean.
The Warrior brave and true.
We fight, we love, and then we kill...