Sure it's safe, you can connect up to (135 ) SP120L fans to that motherboard !
(Just kidding, POINT IS YOU NEED TO CONTACT THE MANUFACTURER /DOWNLOAD THE OWNERS MANUAL OF THE MOTHERBOARD AND SEE IF IT HAS SPECS on HOW MUCH CURRENT DRAW the HEADER is RATED TO HANDLE. I'D SUGGEST YOU E-MAIL THE MANUFACTURER (Tech Depart, NOT SALES) vs or in addition to calling them so you will have something in WRITING.... On the phone you may be told you can run 135 SP120L fans VERBALLY.
If rep says header will do 2amp, have him put it in writing.)
With that info in hand you compare it against what the fans are rated at trying to keep the load at about 50%.
also, since my rear system header on the mobo has a 4 pin, that means i can control the RPM right? while the 3 pin one will always run at full speed?
Depends... 3-pin may be variable /programable as well. Go into the BIOS and or Download whatever apps mfg might have to control fan speeds.
3 pin, vs 4-pin
4-pin should use what is called PWM.
The speed of 3-pin fans are controlled via VARYING the supply voltage to the fan. More voltage spins the fan faster.
4-pin fans receive FULL SUPPLY VOLTAGE. "Speed" of 4-pin fans is controlled by a controller chip embedded within the fan which turn the fan on and off rapidly. (In pulses)
(To illustrate plug in a vacuum cleaner, blow dryer etc and rapidly turn the switch on and off... Notice how you can speed it up or slow it down)
The 3 pin way is the old way, the 4-pin control scheme is the new way.
3-pin fans are generally cheaper. *
4-pin fans - have that embedded controller chip in them, which adds to the cost to manufacture.
A 3-pin fan will plug into a 4-pin fan header, result is it will run at full speed.
A "powered" PWM comtroler which gets it power directly from the powersupply, can control /regulate 4 and 3 pin fans AFAIK.
*Note, Depending on where 3 and 4 pin fans are in the "Product Cycle".... (Inverted Bell Curve) 4-pins might be cheaper. Buy it early in lifecycle price is high, when it gets popular price drops, as it is phased out price may increase (somewhat)... as the item (whatever it is) is "rare"... and they know they've "got you" if you need to "match" something up.
Trick to buying is not to be the first guy to the party, nor the last.