it wouldnt be wise to tap into any wires that go into the cluster, especially power wires. i have seen things malfunction from doing this.
your 12v source needs to be an "acc on" source so basically anything that is off untill your turn your key forward will work. your best bet would be tapping directly into a what i think in a civic is a very large yellow wire running into the underdash fuse box. doing this will assure you dont have to worry about possible power source interuptions or stealing power from something else.
as for the rpm source wire in the guage cluster, once again you can use it but i wouldnt. your best source for a good accurate reading is going to be the ignition coil itself or the blue wire running out of the distributor. readings from the ignition system are sometimes "cluttered" from electronic interference even with the shielding on the wire. taking your reading from the source or as close to it as possible is always the best bet. just buy a spool of wire and lengthen the wire coming from the tach or shiftlight. make sure you shrink wrap it so you dont defeat your own purpose.