TCJ4, I have been modifing the Intex pools for a good three years. Make sure you take a close look how others have modified their pools when adding a skimmer like the "real" pools have. Personally, I would make sure (like most of the threads so far) you have valves on each return so you can have the ability to turn both off to work on pump and/or filter incase you need to or to select which one you would like to use. I would also recommend to help out keeping the pool clean, to put the cover on when not using the pool. With the valves you can turn off the skimmer when the cover is on. The key is education, show everyone the proper operation of the pool and why we turn this valve on or off.
Additionally, because I went to the 2500gph cartridge filter upgrade versus the original weak pump on my little intex......congratulations you now MUST educate your pool users on the suction hazard of pool returns. Especially if someone forgets to have both the skimmer and that screen looking intake open, I promise you with just the screen looking skimmer open, you have a major suction hazard in your pool and that upgraded pumpl!! Someone can get hurt or at the least, surprised when they get sucked up into it. Just make sure pool supervision KNOWS what is going on and that both "intakes" are open. OK? Been there done that. Fortunately, we just had some "surprised" kids and no real damage or life threatening situations......your mileage may vary. For planning I would keep the new skimmer and that "factory" screen intake two to three feet from each other.
Plumbing up my Intex this year, I will have one and only one "return" putting water back into the pool. You will have alot of flow with that pump.....plenty to keep things moving. If you really want to "stir" the pool up, consider purchasing one of the venturi type returns available that will magnify the effect of the return jet AND is built with a 45 degree or so curve in it to get the flow going to mix stuff up. Got mine on Amazon and it has regular 1 1/2 plumbing fittings.
I would recommend you go with 1 1/2 inch PVC pipe AND I would suggest you have flexable PVC somewhere in the mix so that the hard plumbing has some flex between it and your pool. You don't have a stationary pool, it is going to flex, so plan on it. Again, someone on the forum here has a nice picture to show you that setup. Just keep looking and it's been referenced.
Dan's link to everything is a supperb primer on this stuff. I will repeat again that the 1 1/2 electrical conduit fitting makes everything nice and easy.
Be a hero (like I will this season) and add the fountain return (and associated valving/plumbing) to your Intex pool to really up the fun factor. That is THE absolutely best idea for an Intex pool since the whole hard plumbing thing was developed.
Good luck with the pool.
Bob E.