The short version: Q962 = 'must have', Q963 = 'almost indispensable', Q128 = 'should have', Q961 = 'cool, but can live without'
Longer winded comments: The Q963 is an awesome addition. The Q960 has independent trigger outputs at each stage, but no summed trigger output, so using triggers directly from the Q960 can require a lot of patching. The Q963 uses an internal cable to bring all of the triggers to a single bus allowing each step to be routed to one of two outputs. The only downside to the Q963 is that a trigger can't be routed to both the A and B output channels on the same stage, but it does allow addition of rests without changing the setting on the Q960 itself, and gives you access to all the stage triggers without a spaghetti mess of cables. If you're thinking long term, and may expand your system later, then up to 5 Q963s can be daisy chained together (more behind pannel cables) and with multiple Q963s you have complete flexibility to use any combination of triggers.
The Q961 has a couple of uses, the primary being combining triggers from the Q960. Note that the Q963 accomplishes this as well. The Q961 also allows for timing adjustment of triggers connected to the column b inputs, this can produce really cool effects, but isn't something you're likely to use all the time, and the same effect can be had using a gate delay module (I use a pair of STG Soundlabs dual Yves Usson Gate Delays)
You might also consider the Q128 Switch, while not technically a Q960 accessory module, it can take inputs from the Q960 oscillator out and the Q963 or Q961. You then get distinct triggers out from the Q128 rather than full stage triggers, this might not seem significant at first, but consider that adacent triggers from the Q960 will result in a single longer trigger.