The weakest engine you're considering will pull a Rascal 40 ARF, a Venus 40 ARF, or an Ultimate Biplane .40 around with plenty of authority.

I flew my most recent Phoenix SeaBee ARF with a Magnum XL .52 RFS, which is considered the weakest of the .52 to .54 four strokes. With a Fox Miracle Plug and 10% Wildcat fuel, it was turning a Master Airscrew 12x6 at 10,000 rpms and the plane performed wonderfully. I enjoyed terrific vertical thrust, knife edge flight across the horizon, and snap rolls so fast they were a little scary!

Any ball-bearing .46 2-stroke would be plenty of engine for the three airframe categories you're considering. Any .52 4-stroke would be plenty of engine as well. Proper tuning and propeller selection go a long way toward maximizing your engine's performance.

The .56a 4-stroke from O.S. would be a significant investment up front, but that would literally be an engine you could be flying a decade or more from now. I think the .55 AX would be a great engine on any of the airframes you're considering, but I don't think it would be an easy engine to hide under the cowl of a Rascal .40 ARF.

If you love O.S. Max engines, I don't want to try to talk you out of them. I just can't help but think that the Saito .62A or Thunder Tiger F-75s four strokes might be smarter buys for the money. The Saito .62A is readily available at $209.99, which is about what the O.S. Max .56a 4-stroke sells for after discounts. At $169.99, the TT F-75s is barely more expensive than the O.S. .55 AX 2-stroke.

At the end of the day, either one of the engines in you poll would work great, as would the O.S. FL-70 4-stroke. Good luck and good shopping!