1. autoptimize does not interfere with wp_head, but all javascript in head (put there by plugins using wp_head) is aggregated and (by default) loaded later. you could either exclude the slider's javascript (and jquery as well, as that'll probably be a dependancy) or you could "force in head"
2. no definitive answer; depends on the amount of disk space you have available to you. if you have a large number of autoptimized (js-)files in your cache, that could be due to some random (or page-specific) javascript being added (probably inline) by a plugin. in that case you'd be better off excluding that from being autoptimized (also beneficial to your visitors, as else they would have to load another autoptimize_xyz.js file with each page request).
hope this helps,
frank