Small should be different than Medium for damage and carrying Bulk. I like playing gnomes and have accepted the disadvantage to attacks that that brings in favor of the advantages of being harder to hit and to spot. If there is no difference, than why play the smaller characters.
Flat footed can also come from climbing, acrobatics, various feats, spells, and class abilities. It definitely isn’t a surprise round-only thing.
I don't see any reference to flat-footed from climbing. I remember some reference to loosing Dex adjustment to AC while climbing. Please provide any references you find. I have searched and not found others, besides the Surprise Strike.
Also the basic condition of a weapon that the victim is not aware or is a given if you are invisible and they do not have a way to see invisible. Sleight of Hand probably has a +20 circumstance bonus when you are invisible.
The flat-footed condition is almost exclusively the surprise round. It seems very arbitrary to restrict it to the surprise round. The flat-footed condition is not caused by other things like Feint or things that remove the dex bonus to AC.
Having both Quick Draw and a Glove of Storing makes the concealed carry not an issue.
Can these be used in combination in the middle of a battle? If a rogue has a concealed weapon and does a Surprise Strike on an opponent that is not expecting the strike of that weapon, can he do Max Sneak Attack damage on a successful hit? While Underhanded state: "if she makes a sneak attack during the surprise round using a concealed weapon that her opponent didn't know about", the key part in question is that the "surprise round". The key feature or the surprise round is that the opponent is "flat-footed". And that is also a feature of the Surprise Strike, even to the point that: "the target is considered flat-footed regardless of any class features or abilities it might have". So even a equal level rogue could be caught flat-footed by the Surprise Attack.
Would it work if the rogue was invisible? Or is invisibility not needed?
As long as the skin of a creature is not slimy or slippery, can a character climb up a creatures back that is 3-4 sizes bigger? For example could a gnome climb up the back of a T-Rex? The slippers do not require a climb check or skill roll. And you don't need to have your hands free. So it seems to me that one could stand on the back of a T-Rex and be safely out of biting range.