Good work. Well laid out. Glad you are safe, so far. If those are plumbing fittings, your hydraulics guy has misdirected you. A gauge in itself is safe to the pressure on its scale, regardless that its parts may be brass. There is no real fluid flow in a gauge, so the passages can be very small. This translates to very little internal area for the pounds per sq inch(area) to push against. So even with high pressure the forces are low. I have a 10000psi gauge with a brass nipple. It is the fittings used to connect the gauge where you can get in trouble. Even tho they are a stronger material (debatable with cast iron tho) they have much more internal area, thinner walls and are more brittle. Plumbing fittings are rated at 150PSI. This is verry conservative, but I would never rely on them at a pressure higher than that where the nature of a leak may be dangerous. Like steam - - hot, or a high pressure oil squirt. These fittings can be damaged easily and it takes NO damage to make it likely that they will soon fail when used at 10 or 20 times their rated pressure.
As far as your filter - it should be fine. Unless something pretty bad comes from your tank, the limitation of one pass thru the sytem is pretty good. A suction filter would be bigger because more surface is needed to pass the flow at the low differential pressure available. It would be used before the pump. The two types are not interchangeable because of different bypass pressure settings.
Larry
