Before I switched over to DCC I ran MTH PS2 engines on the layout in conventional mode. You need to make sure the switch on the tender is in the correct position. I used MRC power packs straight out of the box and the engines were straight out of the box. There were no problems or continuous bell or anything in this setup. Polarity switching through reverse loops on the layout worked the same as any other engine. I think the sound chuffs played regularly, but you might need a button box to make the bells and whistles play.
One thing I did find strange is in order to get the engine to start I had to pull the lever on the power pack up to high voltage very quickly, then ease the voltage down quickly again and then the engine would start up slowly and then act like a normal DC engine. You would think that if you jacked the voltage up high that the engine would take off immediately, but this is not the case. It starts out slowly. Not too big of a deal once I realized it.
Yes, if you gut the engines you can wire them to work on straight DC.
I still have the power packs hooked to the layout on a toggle and can still run those PS2 engines or any straight DC engine on the layout but I typically run DCC now.