Hey VapingLobster looks like everyone pretty much beat me too it huh? To recap though there is a screen in the center tube to catch any significant debris from falling deep into the core and anything small enough to make it through doesn't linger long. As far as charring goes we haven't had any real charring of the wood in the new insulated core pieces. In any event as I think someone pointed out above the core is completely sealed off from the wooden body so even if charring does occur nothing get into the air path.
There isn't any functional difference between the full size pieces (like a Queen) and mini pieces like Twigs other than physical handling characteristics (ie small is sometimes easier to mate to a waterpipe) and some potential durability issues (ie thicker wood generally is more durable).
There is a small (unnoticeable to many people) heat difference between wood species with the Dogs made of lightweight woods being slightly (and I mean slightly) more hot, this is at the expense of comparable reduction in durability because of the softness of those woods.
Not sure if PD stems will fit, maybe someone else here can answer definitively.
What specifically do you mean re: replacing one of the glass stems for a fatty? If you mean 1 SGS (simple glass stem) for one WPB or XL1or2 then no sorry too big a price difference. On occasion I will give credit for the SGS stems when people don't want them so they can use that credit towards a 'fatty'. If that's what you're interested in just shoot me an email or message for details.
@ mwape - heya dude I haven't tried a Solo so can't really comment on that but yeah for best effect the UD bowls will need stirring these days. When the cores (older ones) were longer (including both the center tube and the core body) the hot air was more diffused when it reached the bowl but with the new smaller, insulated core with the shorter center tube the air is like a super heated jet in the middle of the tube so heats more in the center of the tube. Not sure how that compares to the Solo but that is what's going on in/with the UD.