Wow, a 150 is a great tank. I am assuming that it is already running. I thought I would comment on a few things not in your specific question, in case this is posted on the site and others read it.
Water is heavy (period). With substrate, a 150-gallon tank will weigh in at over 1200 lbs. Placement is, therefore, critical. If the tank is sitting on floor joists (not a cement floor) care must be taken. The typical floor joist set-up is a separation of 12" between joists on newer homes. This can carry a load of 40 lbs per sq. foot.
So a quick calculation (not sure if measurements are exact) 6 foot X 1.5 foot = 9 square feet. If the tank is lying across the joists this means that those joists can handle 9 x 40 = 360 lbs (not enough!). The danger is not that the tank will suddenly crash through your floor and flood your basement, but rather that over a few years the joists will sag. This sagging will cause stress on the tanks seams, and eventually the tank may leak, crack, etc.
All of this greatly depends on your tank placement. Placing your tank near the outside wall (above the foundation) is quite different than placing it running with the joist, closer to the center of a room.
Regarding your specifics: 400 gph of filtration is a good flow for your tank. A
general thumbrule for freshwater tanks is 1-2 times the tank volume per hour.
Salt tanks require a little more, and reef salt tanks even more, but you are
I would have a filter unit at each end of the tank. It would be easier to reach them for maintenance (rather than both of them in the middle), and, being on opposite ends of the tank, they will provide better cross current flow (to help stir up sediment on the bottom).
One heater would probably be fine for your tank. Place it in the flow of one of
the discharges from the filter. I would buy a high quality submersible heater.
If you have the funds, a cheaper back-up heater would be a wise investment
(just something you keep below your tank, just in case).
As for fish, wow a 150! You have plenty of room. Just don't stock it all at
once. Every time you add fish, your tank will go through a mini-cycle while the
bacteria population (or biomass) catches up to your fish population.
Although some people go for more, a general thumbrule for freshwater tanks is 1 inch of fish per 2 gallons of water. A 150-gallon could be a great discus tank, or predator tank, or breeding tank for cichlids, or even a brackish amazon tank ... lots of possibilities. Take it slow; introduce fish slowly. Watch for trouble.
Ruby would probably be a better gauge on freshwater fish temperament than I. I always liked the freshwater sharks; they can get big, but you have a big tank.