I think it's funny that you called on me specifically and got answered by the two people I would have suggested you ask.
I do have a few thoughts that I can share though...
#1
You will probably struggle a bit at whichever game you learn
second. It does take a little bit of effort to adjust to the opposite location of the panels between the two games. I know I stomped metal more than a few times when I first started playing PIU regularly. Eventually you just get used to it and it just becomes another skill like when you learned the first game.
One night MonMotha and I alternated between DDR and PIU for each song. We'd play a song on one game and then immediately switch machines for the next song. Neither of our heads exploded and we played both games fine.
The timing windows are definitely much looser in PIU, so it could affect your PA in DDR. PIU seems to be far more focused on pass/fail rather than precision. The extremely difficult stuff in PIU is much more difficult than the hardest stuff in DDR and it's expected that you will rely on a certain amount of "slop" to be able to pass. I don't really know any PIU players that count S grades or SDGs or anything like that.
The patterns and play style between the two games are pretty different, so I don't know that either one will directly help you play the other. However, what MonMotha said about building stamina is quite true. PIU has songs that make DDR's "stamina busters" seem like a walk on the beach with your girlfriend. Wait...that could get your heart going, too....a walk on the beach by yourself.
I would say that it's certainly possible to be equally good at both games. TaQa is incredible at DDR and was able to gain a massive amount of PIU skill in a relatively short time last summer...though I don't think he's kept up with it (PIU).
Personally, I just started playing DDR semi-regularly again after playing Pump almost exclusively for the past year. My PA has never been very good, but it actually seems better now. Go figure.
#2
Passing songs on CZ or NM seems to be considered good on PIU. Passing with stage break on, even better. When comparing two people who can both pass a particular song, letter grade seems to be the next differentiator. After that, probably fewer misses.
Freestyle is awesome! I find freestyling PIU seems more fluid and natural than DDR.
#3
Pump Haven is the only one I'm at all familiar with, but I don't really frequent it.
#4
Flakk is right on saying to work your way up from the bottom. The biggest mistake I see people make is believing that since they can pass 9s and 10s on DDR that they should start there in PIU. They are different games with different skills. If you're a DDR player, you may not want to start at level 1, but 2s and 3s might be appropriate for at least a song or two just to get used to the different stage layout.
As for Freestyle, it just takes practice. I don't think there is any real trick to it. It seemed like a more difficult transition for me (DDR > PIU singles vs. DDR > PIU doubles), but with practice I'm very comfortable with it now.
#5
Flakk already answered. I didn't even know those were out there by themselves...good to know!