yes u can provided they blend in well with ur site background or font color otherwise they will act as major distractions if u have sites with different background and different font colors
you can use adsense on any website that you manage... provided that they do not contain adult or illegal material
you can ad the code to as many sites as you want. but it would be ideal to generate the code for each site and save under different chanel name. that way you can track which site is generating the clicks and paying how much. also, you should blend in your ads with your site layout to maximize thge CTR
you can but is better to have different channels ... to know more details about what's happening with your account
yeah...sure man...you can put your adsense code in all the site you have right now...but dont against adsense TOS