Yeah mate i'd fit the ones i could in the factory positions so it looks neat (water temp, oil press), and trans temp sender can be mounted on the side of the trans sump pan for most accurate reading. Haven't fitted an engine oil temp sender before but assume if there's room internally then engine sump would be best too.
If you wanted to run factory guages/lights aswell as your additional ones, Repco etc have T-fittings to fit both senders. One thing to note is if, say, both temp guages are electric, putting both wires on just one sender instead of fitting two separate senders can cause both guages to read incorrectly.
