Keep the dirt you have taken out of the hole when the post was set because it can be utilized to level up the bottom board.
If the property is on a gradual slope, calculate each channel height fr each post before fastening the channels to the posts because even, gradual, stepped panels look much nicer when finished than different pane heights throughout the fence. For example: if the property is on a gradual slope downward try and drop each panel by 1" or 2" all the way throughout the line rather than dropping the first panel 1" then the next 3" then the next 2" etc. this will not look as nice as if you do them all the same. the same theory applies to climbing a gradual slope.
Also try and keep the sequence of your dropped panels or raised panels the same throughout the line. For Example,if you can keep the first two panels level and then need to drop down, try and drop 2 panels, and then 2 panels again etc. This will also keep the fence looking nice and linear when completed. If Possible, Try to avoid' dropping 2 panels, up one panel, drop three panels, drop on panel etc. This would tend to make the finished fence look less linear when completed.
If you are using standard 5/4" x 6' x 6' deck boards then you need:
13 boards stacked for a 6' high fence section.
15 board stacked for a 7' high fence section.
17 boards stacked for an 8' high fence section.
Recommended is 6 foot wide sections and if deck boards are not available in 6' then 12' boards cut in half are ideal.
6' between posts is highly recommended in order to keep the boards from warping or sagging as the they weather.
The Horizontal Slipfence system for residential use was designed to use 5/4 Deck boards which are called 5/4" x 6" x 12' - and then cut in half for 6' sections. However, The actual dimensions of the Deck board recommended is 1" x 5.5" x 12' cut in half to 6' each.
Thus when calculating the qty of Deck boards needed for your fence you simply multiply the number of sections by 6.5 for a 6' high fence, 7.5 for a 7' high fence and 8.5 for an 8' high fence.
The Slipfence horizontal system was designed for wood deck boards not composite boards.
Composite boards could work but not recommended as there are so many variations of composite boards. Also not all boards have 2 sides that are finished so both sides of the fence are presentable. Composite boards also tend to bow toward the sun on hot days and could potentially sag in the middle over time.
Yes there is a new Horizontal Cap rail available now that is 84" long so it can be cut to any length under 7' long.
This cap rail fits over the top of the top deck board snuggly and is trimmed to the same size as the top board with a metal blade on power saw or hacksaw and secured either with couple of wood screws, (that came with the horizontal channel kit), tapped into the top of the rail on both ends in the Horizontal channel kit