I highly recommend locating your fuel and battery in the very back -- building platforms (preferably enclosed with top access). You have the space over those toons that stick out, so why not use it.
I don't think weight distribution will be a big issue, particularly if you keep the small/light motor in your pics, and also if your pontoons are 24 feet long, as you implied.
Your original post said: "the pontoon is a 20' overall with a 16' playpen."
I'm not sure of the official definition, but doesn't a "20 foot pontoon boat" mean its 20 ft front to back of the pontoons??
If you have a 16 ft playpen, and 4 foot floor space in front of the playpen as it appears in your pics, and another 4 feet length of pontoon sticking out back behind the playpen again as it appears in your pics, then doesn't that mean your pontoons are 24 feet? Maybe I'm missing something??
This is personal preference of course, but unless you have a good reason to leave the front of the playpen where it is, I'd also recommend moving it to the very front. It will make a huge difference to how it feels moving around the boat. Why did you say you couldn't move the playpen forward? I have an 1989 24 ft pontoon and the front of my boat was similar to yours, and I did extend the railing to the front of the boat without much trouble. I also put in double doors (gates) at the front which provides a nice wide 4 ft opening with both doors open..
Most people get pontoons for the space, and so why not maximize it. Moving the railing to the front, and putting your battery and fuel tank in the back will increase your usable space significantly. I believe ALL newer pontoons have both of these features -- railing extends to the very front, and utilization of space to the rear of the toons.
I attached a pic of my boat before my restoration (all I have access to at the moment), and it appears the front is similar to yours. But my back railing/playpen extended further back than yours. (by the way, I replaced my back platform located behind the back bench seat, and I extended it and cantileavered it about 12", essentially over my motor - can't have too much space! Makes a good sleeping surface.)
Good luck!