Wall, Protectga, and Shellga stack with everything. that Bartz SSB will stack with all of that, too. a second straight +Defense effect would not stack, tho -- it would overwrite.
something like Hand of the Emperor (+Attack and +Defense) would stack with all of the above, tho. in general, if an effect does two things at once it will stack with effects that only do one of those things. so you could even, in this hypothetical scenario, add in Shout (straight +Attack) and every ability i just mentioned would stack. but casting Planet Protector (also straight +Attack) would not stack further... it would overwrite Shout.
this is why Full Break stacks with everything... it hits Power/Mind/Armor/Mental all at once, so it stacks with all the individual Breakdowns, as well as with things like Wakka's Official Ball SB that lowers DEF/RES as one effect.