Virtually all of today's trainers will fly very well with a .25. The student would have less problem handling the plane with the smaller engine, in my opinion.

When I first started teaching RC flying back in 1971, I guess, just about everyone started with a 3-channel trainer. Sig Kadets, Goldberg Falcons and other trainers did not have ailerons then. People learned to fly on them very easily.

These days with people putting .46s in trainers with flat bottom wings, they fly so fast you need to put in a bunch of down trim to keep them from climbing. When you throttle back to land, the elevator takes over, causing the plane to dive or the pilot to hold back stick. In the early stages of learning, when a student corrects his ailerons, he tends to relax on the elevator causing the plane to pitch down and boink on the ground. This is the reason I recommend putting 2 popsickle sticks under the trailing edge of the wing.

For a .40 sized trainer, if you want a .40, I recommend the much maligned OS .40LA. Sure it's low power, but it is plenty for a trainer, it's easy to tune and it's very reliable. That's the reason they sell so many of them.

If you really want to slow down, try an old timer free flight. Nearly all of the 1930s free flight planes have been converted for RC. Here's the URL for Penn Valley Hobbies that sells the Quaker and Dalaire. A buddy and I have the small Quaker and it's very relaxing to sit back in a chair or recliner and soar around with the old timer.
[link=http://www.pennvalleyhobbycenter.com/]Penn Valley[/link]