Just wondering if a .45ACP cylinder can be reamed out to .45Colt. If so, would the ACPs still work with moonclips? If not, could a .45 Colt cylinder be fit to a 625-4 in ACP? Is this all more trouble than it is worth?

More trouble than it's worth. I'd look at handloading .45 Auto Rim cases with a decent SWC at 900 fps, does almost anything you want.

The problem with reaming out a .45 ACP cylinder to accept .45 Colt cartridges is that the headspace for the rimmed cartridge is greatly excessive...something like more than 0.030". That means ignition will be unreliable. The other problem is that with such a reamed out cylinder, the accuracy with .45 ACP/Auto Rim will be mediocre, with considerable bullet jump from case mouth to chamber throat. What you end up with is a less-than-reliable gun using .45 Colt, and a less-than-accurate gun using .45 ACP. Is that a good tradeoff for being able to fire both cartridges? No, I don't think so.

As for your second question of fitting another .45 Colt cylinder to the frame, the excessive headspace problem goes away, but the new one that develops is that the frame lug will not clear the new cylinder. So, to answer your question, YES it is more trouble than it is worth, unless unlimited money, time, and gunsmith ingenuity are available.