Can someone check and maybe give some tips to a fairly fresh electronics guy.

Projects General Guidance
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I want my Arduino to control 6 different small electronic motors. They are from battery powered fish tank air pumps that run off 2 AA batteries. I only need 1 of them to run at a time so I figured a Demux would work nicely. in reading through other peoples demos and ideas it was apparent that the arduinos ports did not put out enough current to power the motor. so I put together this schematic. My problem is selecting the correct transistor. my guess is that 100mA is enough current to get these guys running, they are you''re standard can shaped motors that get used in every basic project.

Is there any good way to measure the current draw? What transistor should I select?

Once that is selected, I figured I could use PWM to control the speed of the motors, essentially my program would select a particular pump and the pwm would control the speed. This seems reasonable to me if its only getting high pulses for 80% of the time, it will run slower (probably not 80% speed, but precision is not an issue here). any problem with that logic?

any issues with my schematic?

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Your schematics look pretty good to me.

The power supply to to motors should really be separate to the Arduino, you have VCC there which implies your using the 5v from the Arduino.

The GND for the motors and mux has to be connected to the Arduino GND. You haven't drawn that.

With a Mega you don't really need the mux, you would have enough PWM pins to drive the transistors directly.

A decoupling cap (0.1uF) should normally be placed across any logic chip, as near to the GND and VCC pins as possible.

Don't the motors have their current on a label?

Rob

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What multiplexer chip are you using? Is it a latched one? If not it is likely that it will default to a logic one output when not addressed. This means all the motors are on except the one you are addressing. But it depends on the chip.

Is there any good way to measure the current draw?

Yes it's called a current meter.