I had an advertiser contact me for an ad space and was wondering if it is against the terms of Adsense policy to display them and yes they advertise with Adwords also but have never seen their ad on my site through Adsense. Again they contacted me first and do not want to place their ad until I am 100% sure. Also the ad they want does not look anything like Adsense ads. Thanks
Yes you can place other advertisiers on pages with adsense ads. You can filter your advertiser's adsense competitors too.
Go for it. It's not against the ToS and you can often bank more money by cutting out the middle man (in this case Google).
You can do it. I have the ads from 4 ad networks including Google Adsense in one of my site. I have not faced any problem before.