If I wanted to put multiple devices on one serial port, is there any reason why I couldn't connect them up to FETs and individually turn them on and off when I wanted to grab data from each sensor? As long as I make sure only one sensor is powered at the same time I should be fine right?
try a web search for rs232 multidrop
do the serial devices already exist? if so are they RS232 or 5V serial?
if not consider using RS485
Powering the devices off might result in parasetic powering via the TX line of the Arduino or from the TX line of the device that is "on" .
I think it highly depends on how those devices work. Please provide links to the sensors.
If you can control when they send, you will be OK with e.g. two analogue multiplexers. If you can't control that, you might get corrupted data if two are sending at the same time
You can connect several slave devices (such as other Arduinos) to an Arduino serial port if you use diodes to ensure that the TX from the slaves are isolated from each other. Obviously your software will need to ensure that only one slave talks at any one time.
The diodes should be organised like this
The master TX can be connected directly to the slave RX pins and each slave will receive the same data.
Oh come now!
What are these "sensors" that put out serial data, is it asynchronous serial data or some other form, and how do you propose to turn them on and off?
It is - as ever - a waste of time making suggestions when you have not adequately described the "problem".