All depends how you are wiring them but as you have 6 lights and 3 relays I'll assume in pairs, will also assume 100w bulbs as you may want to fit them later anyway.
Don't use figure of eight stuff, buy proper auto cable, you'll waste loads of earth wire with figure of eight as your feeds will need to be longer. Also everything will end up the same 2 colours. Buy from elsewhere an get lots of colours so you can see what you are doing.
For all cable, switches, fuses etc have a look here -
http://www.vehicle-wiring-products.eu/VWP-onlinestore/home/homepage.phpEach 100w bulb draws 8.3 amps so the feed from the relay to each lamps needs to be capable of carrying at least that preferably more. Ref 16. 32/0.20mm, 1mm2, 16.5amp @26p/metre
Earths from each bulb need to be to the same spec as the feed
Each relay needs a feed through a fuse capable of supplying 2 bulbs so minimum 16.66amp - Ref 33 . 44/0.30mm, 3mm2, 33amp@ 50p/metre
If you want to bring a common feed up from the alternator/battery for the fuse box it will need to be good for 50amp so Ref 80. 80/0.40mm, 10mm2, 70amp @ £1.38/metre
You'll also need a switching circuit, this carries next to no curent so can be the thinnest stuff - Ref 11. 16/0.20mm, 0.5mm2, 11amp. @ 21p/metre
You don't need a tell tail light but if you fancy one it can't harm. They do need to be wired so that they can only come on with main beam to be truely legal on the road. A three position switch cn be good for this. Centre off: Up Overide (comes on on their own) - Down Road setting (on with main beam only).
I'm not sure if you are doing but I would put them on three separate switches so you can choose which you want on when.
If using 100w bulbs, whats your alternator like 6 x 100w + 2 x 60w mainbeam is 720W or 60amp. Which probably doesn't leave you much to capacity in your alternator after everything els is powered and alternators don't last long running at 100% all the time.
Suggest
BRAIDED SLEEVING for keeping things tidy under engine bay etc, needs a bit of heat shrink over each end to really tidy things up.