Most of my audio work has been pro sound and not home audiophile. The same physics apply though. If you are not running LF to your surround speakers, 14GA lamp cord up to 60' long is plenty overkill up to 300 watts of power. Its cheap and will last essentially forever.
Audio cables should have good build quality with decent connectors (no gold plating please). They run about $1-$2/ft, not $20/ft "premium quality" cable. The extra $19/ft covers their extensive marketing costs
With line level outputs you can connect as many amplifiers as you wish to power extension speakers. Pay careful attention to matching speaker ohms to amplifiers capability and resist the temptation to run speaker wire longer than 60'. The closer the amp is to your speakers, the better.
Good luck with your theater project.