Design help for switching a 5V-supply: FET or BJT, relay or not?
The tutorial you've linked is way too basic. I'd recommend you to take a look at this one, which has deeper detail without becoming excessively complex: MOSFET as a switch - Tutorial

Q1:

If my understanding is correct, your application requires cutting off a rail that supplies 5V to a load (or more than one) connected to ground.

If you want to do that, you'll need a high side switch. A P-channel enhancement mode power MOSFET may be your best option, as there are plenty of devices available with logic-level compatible gate threshold voltage (a very convenient feature).

Check Vishay Siliconix, International Rectifier or any other manufacturer MOSFET Selection Guide for a suitable device.

This one Si4477DY could be good your application due to its very low RdsON (&lt; 6 mOhm i.e., 12 mV drop at 2 Amps) and because it will be deep driven when grounding its gate (you'll have Vgs = -5V). You could also trade off some of its capabilities for a lower priced or more conveniently packaged device, it's up to you.

If you need to an extremely low RdsON, you could opt for a N-channel MOSFET, but then you'll need additional circuitry to drive its gate (for example, LTC1154) and complexity can escalate pretty quickly. I'll recommend against it.

Q2:

You can switch an AC load through two MOSFETs, but be aware that their body diodes will be conducting alternatively thus dropping some voltage. For a 12V AC load you may be better off just using a relay.