I'd go find the manual for the trim tabs and see what it calls for for oil. Keep in mind that there are ISO and SAE weights for hydraulic oil, and most are sold based on their ISO weights now. Just be sure you are getting the right one. A quick google search will surface conversion tables so you can cross from one to the other.
As for automatic retract, I suppose it could be done as you describe, but there are a lot of things to check, like normal operating pressures for each system, max allowed pressures, oil types used, etc. I probably would not interconnect them.
But it's kind of a neat idea, so what I might do is look at controlling the tabs electrically. It's usually not to hard to get a switch that signals reverse. It could be done mechanically off of linkage, with a pressure switch in the reverse clutch circuit in the gear, or some shift controls have outputs that will signal it. Then use that to operate the tabs to retract them. Or just make a little sign and stick it by the gear shift control.
