It could just be that with the combination of the new piston rings still seating and the fact that their sealing/compression is now going to be better, the old starter just isn't going to turn the engine over in the same manner as it did before the engine was rebuilt. Was the old starter reused, and if so, how many miles does it have on it?

It's possible that as the rings seat in the resistance they have now (being new) will diminish a little and the old starter will be better able to turn everything over, but if it doesn't get any better, maybe a new starter would be in order. I know I had the same issue on a Mustang GT years ago anyway when I rebuilt an engine using Total Seal piston rings.

Unfortunately, that wouldn't explain the idling differences, but for that, you may want to check that the coolant system was properly burped when it was refilled after the rebuild and that there are no air pockets currently trapped. Something like that could cause intermittent coolant temp spikes that might affect the idle.