How to measure current and voltage and rpm with daq device

i am measuring current and voltage and am wondering should i use shunt resistors or current sensors hall effect sensors. i have a pci 6221 and a scc-68 breakout box. what specifications or size should the shunt resistor/current sensor be or should i use another device to measure the currrent and voltage. do i need any other safety device between the resistors, current sensors and the scc-68 breakout box when measuring the current and voltage. will the daq card pci 6221 pick up a signal from the resistor. what terminals should the wires from the resistors and current sensors be connected to in the scc-68. i am using a proximity switch to measure rpm of a motor. should the proximity switch be 2 or 3wire connection. should it have an analog or digital connection and is a power supply required to power the proximity switch and should it have an npn or pnp connection

hello,
i was going to use a 20amp 50mv, or a 20amp 100mv current shunt and connect wires directly into the AI input terminals of the scc-68 from the current shunt would these be suitable. would current shunts such as these be suitable to use http://uk.farnell.com/elc/sh10020/shunt-sh10020-20a-100mv-1-class/dp/1319576 or http://uk.farnell.com/datel/3020-01098-0/shunt-50mv-20a/dp/1339338
is it ok to use either a current shunt or should a hall effect sensor be used such as this http://ie.farnell.com/honeywell-s-c/csla2cd/sensor-hall-effect/dp/1082269 . which of them would be more accurate or are both of them fairly accurate.
when i am measuring voltage can i connect two resistors between the positive and negative of the wires going to the battery and connect two wires from either side of the resistor directly into the analog inputs of the scc-68 and measure the 12-15 volts directly would the pci6221 and scc-68 be able to measure the voltage drop across the resistor.
i wanted to measure rpm also does it matter whether the proximity switch has 2 or 3 wires and should it have an analog or digital/frequency output for cponnecting to the scc-68

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