As long as its nit light or hollow its good enough to cook with. With wine I assume its the same, discard any bad or rotten and only use the close grained ones
Rhubarb should really be past it's best by this stage of the year, ours is all but finished and the stalks that are still on it are quite small. But then we are quite far north here and it's got quite cold recently.
At this stage rhubarb is way past its best.
The major problem with haervesting rhubarb after August is that you weaken the plant and hence it wont grow as strongly next year.
IMO Best to leave it this year and have a plan for next.