I think Big Al's has some of the better prices--I've never really shopped for fake plants, though, so can't offer much help.
Male guppies will tend to chase each other around, but without a female to 'encourage' them, they are fine.
In a 50? I would go with a trio of rainbows, or gouramies. Some of my favorite fish. In the 30, if it's a well covered tank, some of the smaller hatchets would help fill in the top of the tank. They'd work in the 50, too, and you could have a group of 4-5, instead of the rainbows or gouramies.