A pot(entionmeter) is usually used as a volume control in an amplifier, etc. It is a low current device. A typical radio shack pot will carry a watt or two. A rheostat is usually associated with controlling higher currents. They are significantly more expensive when you get above 10W (which will control less than a 1 amp train or burn up trying!)
Another option would be an adjustable wirewound resistor (google it). But again, they get expensive for higher loads.
In the long run, two of the variacs I pictured would probably cost less! And as mentioned above, the transformers you use will be getting a nice pure sine wave to plug in to, so any transformer should work well, (just at a lower voltage in which will result in a lower voltage out.
Another thought. If you're using Z-1000 type transformers, don't use the supplied brick. Instead, use a (fused) heavy duty 12V AC transformer. The Z-1000 will get 12V in instead of 18V, so the top output will be about 2/3 of normal.
ps. see what happens when you type S-l-o-w-l-y!