I have some old Tyco rolling stock that doesn't want to connect very well or fast to new rolling stock. Can anyone tell me if I can easily remove the old couplers and add new ones? If so, what coupler types work best?

I have the Rail Chief and I've added several feet of track. I'm still running the power supply that came with it, should I upgrade the power soon? And should I start to add wires to the track?