The problem with your numbers is that the 22gpm pump unloads at around 500psi and is only pumping 6 or 7gpm at high pressure. The lower flows and higher pressures will work fine with the smaller engine. Now if that pump was to make full pressure at 22gpm, then your suggestions would be closer to correct. Also. a lot of two stage pumps do have the capability to adjust the unloading pressure, but I don't think any of them will allow the full flow at the high pressure adjustment.First 22gpm needs aboit 19hp at 3000psi. If your running a 2000psi system your 11hp would be the smallest I'd reccomend. I currently have a 16gpm on a 9hp engine running 3000psi it's about at its limit. Your rule of thumb is 1hp per 1gpm @1500 psi. As these are 2 stage pumps you can half the equation. So the math comes out to just over 38hp for full flow @rated psi. Half that for a 2 stage pump give you about 19hp.
Also have you talked to any local weld shops about a tank? I have a 10gallon tank on my splitter and it cos $30.00 for my local shop to cut and bend the steel for me. All I had to do was weld it up and paint it, and install my fittings/ suction screen in the tank. Wouldnt hurt to call around and ask.