the supply voltage is -0.5 to 7 volts.
These values are "limiting values", which other vendors more often call "absolute maximum values". This doesn't tell you what voltage to operate the chip at, it tells you what limits to remain within to avoid damaging the chip.
To see what voltages you should operate the chip at, look at table 5, "recommended operating conditions." There it gives the limits of 2.0 to 5.0 V for regular operation. All the performance specs in the datasheet will assume you operate the chip between these limits, not the limiting values limits.
The supply current has a limiting value because the total supply current will be the static supply current (given in table 6) plus dynamic current that depends on how often the outputs are switching, plus whatever current is delivered by the output to the loads. This puts a limit on how much total load can be put on all the outputs.
what is the difference between output current and supply current?
The output current limit is for any one individual output.
The supply current will be the static current, plus switching current, plus (the sum of all the) load currents delivered to the loads through the outputs.
So if you have more than two outputs delivering the limit of 25 mA per output, you will exceed the 50 mA total supply current limit.
The ratings on page 3 list the supply current as 50ma, but the supply voltage is -0.5 to 7 volts. Does this mean I must limit the current to pin 14(in this case) to 7 volts, and the current no greater than 50ma?
You should use a Vcc voltage according to the recommended operating conditions, not the limiting values.
If you do that, and don't connect too low an impedance to the outputs, the device will limit its current draw well below the absolute maximum suppply current limit.
Regardless of voltage, as long as it is within the allowed range(-0.5v - 7v), the IC will out put Vcc at +- 25ma at any of the output pins?
No. To make the chip operate normally, you must respect the recommended operating conditions limits, not the absolute maximum or limiting values limits.