When the SSD T3 is only plugged in, it needs practically no power. If I copy a large file, then the power consumption on my Surface 2 increases from 8 watts to about 16 W. If a large file is copied internally, the power consumption increases to about 13 W. This means that the SSD needs about 3 watts when reading/writing, and almost nothing else.
Although the performance would allow this without any problems, many game softwares advise against it, or even prevent the outsourcing of files to external drives.
I would therefore advise against it.
Depends on what kind of SSD you use otherwise. According to the manufacturer, the SSD can achieve a read speed of up to 450MB/s via USB3.1.
It is possible to install Steam games, as long as you can define the path for the storage location in Steam. However, I cannot tell you whether this is recommended.