You can use any DC power supply you wish as long as a) it doesn't exceed the voltage rating of the LM, b) has short circuit protection, c) can deliver enough oomph to make the train go and d) is preferably regulated, which you say yours is. There are a few that are using non RailPro power supplies, at least in H0. I'm not to sure what our other large scale users are doing so hopefully some of them may chime in.
The PWR-56 is only 14.8 volts, is that enough to even run G scale stuff? (and yes, you can run more than one PWR-56, you just need to break your track up into sections for each PWR-56)
- Tim