The longer the scope of the line (the horizontal distance along the seafloor that the boat is from the anchor) the longer distance it would be lying on the bottom. In most anchoring situations you want the length of line to be about 4x the depth you anchor in (good weather conditions) so that the pull on the anchor will be mostly horizontal not vertical. In deepwater conditions like you describe you can reduce this to 2:1 but you should also use extra chain to help hold the anchor down. You need some chain for most situations to keep the rope from abrading on the bottom. I would suggest 150' of chain and 800' of rope (of suitable type/thicknesses for your boat). Remember that overkill is better than getting washed ashore in the night! I would not use the 1/4" for any anchoring situation where I would be concerned if the rope fails (and if it does you will be without an anchor) so best to just make it into a heaving line or similar.
If you join two lines togeter use a reef knot (square not) or a sheet bend with an extra turn of the bitter end around the "larger" line and back through the knot. If you don't understand this I might be able to post a picture. Most knots reduce the strength of the line by 50% or more of the rated tensile strength. Also understand that splices, chafing, age, sun exposure, amount of use, etc are factors that reduce the strength of the rope. For anchor line you should oversize the line significantly to account for the wear/tear and the importance of keeping the boat from loosing it's mooring.