I do not see anything wrong with housing them in a 150 gal tank. As far as filtration with oscars they must have very clean water or you run the risk of HITH.
Rena makes the X3, not XP3. I used to use them on my 156g and 135g but later switched to two Fluval FX5 on each tank. After using the Fluvals for a couple of years, I pulled all the Rena's out of storage and hooked them back up again. I switched back for their ease of cleaning and they filter just fine. The Fluvals are a big pain in the butt to break down and clean opposed to the X3's. Anyway, my point is I like the X3's and I would recommend adding another one to help filter your tank. I have a wet dry sump on my biggest tank and that's really the absolute best way to go if the tank is drilled.
As far as stock, I think there's enough space for what you want to keep. Also agree with others about the common pleco. Have fun with it and good luck.
LoL whoops, you're absolutely right, Jc...I'm using them on my tanks and don't even know what their called derrrr...
?