Sweet looking setup. Definitely on my "wish list" (which also includes simply having a tractor.
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The standard "black iron" threaded fittings from your average plumbing store will be 150# malleable iron - rated for only 150 psi operating pressure. You may not be having trouble with leakage, but you are also probably asking for trouble. The pipe itself is not the weak point, but rather the strength of the fitting socket itself - which is already under stress due to the wedge action of the tapered pipe threads. It is possible to actually crack the fitting down the socket by tightening the pipe too much. Where you will likely see a failure is one of the fitting sockets actually bursting - possibly in the event of a pressure surge caused by hitting resistance fast and hard?
A good industrial piping supply company can get 3000# forged steel fittings, but they are substantially thicker and may not fit where you have the standard fittings now.
You are right, the steel tubing is thinner wall than the iron pipe you are using. BUT, the tubing, as you guessed, is annealed seamless tubing, whereas your pipe is most likely A53B carbon, which is rolled and welded. The strength of the pipe, however, is less likely to be an issue than the strength of the fitting and the joint. The hydraulic tubing is connected with one of four joints typically, which are rated at much higher pressures:
1) SAE threads (straight threads with an o-ring seal at the shoulder of the fitting).
2) Orbital butt-welded fittings (almost impossible to see because they are precisely welded with a special machine).
3) Swage-style compression fittings (i.e. Parker A-Lok, SwageLok, LettLok, etc).
4) SAE flange (funny looking oval-shaped four-bolt flanges, welded to the tube and using a recessed o-ring seal on the face - usually used in larger sizes).
The easiest of those to use is the compression style, and they can be ordered, along with the tubing and any tools to bend/work with them, through most industrial machine or mechanic supply shops. I believe McMaster-Carr and Grainger carry the Parker line of hydraulic fittings and parts (albeit probably not cheap).
