Yes it is allowed, you can use anything with adsense like, infolinks, kontera, adbrite, bidvertiser.... Just make sure that the other ads that you are using dose not mimic adsense.
Definitely yes. lots use it to complement adsense earnings, i tried .. but ain't please with the results. Too many dictionary links, and info links OWN google search links (WTF!) .. you get paid less than 1 cents for clicks on those. Infolinks aint as good as 1 yr ago ...
Infolinks ads are allowed to be used with Google ads, on condition they look different. "Can I place Google ads on the same page with other ads?" https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=32849