I agree about fluid type mixing, harmless. Most important for new owner of USED vehicle is Why is it low? A small leak? It is a simple design with master and slave being the only devices.Even if you think it just evaporated it would mean the fluid is very old. Get a helper and add a full pint by bleeding it out the slave cylinder at the transmission.
If you are handy you can remove the intake brace (behind alternator) and then snake your arm back to the bleeder valve with a 10mm wrench , otherwise you will need to jack it up and come from the bottom.
Always disconnect battery in trunk before servicing.
Good luck. I have had my 99 since 98. You won't like your Miata if you loose the clutch from a leak.