In the case of glucose, D and L glucose are enantiomers. So can one say that all D and L sugars (of the same sugar molecule) are enantiomers of each other?
Just out of curiosity, why is it that if a sugar is D and it becomes L, all of the chiral centers flip-- is this just nature at work?
That's just the way they're defined. If you were to only invert some but not all of the stereocentres, you'd end up with a different sugar.
I ask because if one is distinguishing b/c alpha and beta of the same sugar molecule, it is deemed as an anomer (which is an epimer which is a diastereomer)?
An anomer is a specific kind of epimer that refers to the two diastereomers that arise when considering the stereoconfiguration of the C-1 position of cyclic sugars. Note that this carbon is not stereogenic in the linear form of the sugar.
What makes the penultimate so special that the whole molecule becomes an enantiomer instead of just an epimer? Does this have to do with how the straight chain becomes a ring structure?
I'm not really sure what you mean here, but I'll have a go at elaborating on what I said before. The definition of whether a particular sugar is D (or L) configuration has to do with the configuration of the stereocentre that is the furtherest from the most oxidised end of the straight chain molecule. The L (or D) version of that same sugar has all of the stereocentres inverted by definition. As I said before, if it were just one or two, etc., then it would be a diastereomer and it would not be the same sugar.
Idose is a diastereomer of glucose as it differs in the configuration around three of the four stereocentres. If we look at the D sugars, you'll see that in both cases, the stereocentres furtherest away from the aldehyde group (in blue) are in the (R) position, whereas in the L sugars that same carbon is in the (S) configuration. If we were to change the configuration of only that carbon in say, D-glucose, what we end up with is not L-glucose, but in fact L-idose. Similarly, if we changed the configuration of that carbon in D-idose, we'd end up with L-glucose rather than L-idose.
Hope that clarifies things for you a little bit.