I know that adsense pays better for the 1st adsense ad found in a webpage. But does adsense pay better for one ad format vs another?
Seeing as you get paid based on the ad itself and not the format of the ad, I wouldn't say that one format or the other pays more. However depending on your site, one specific format might be much more visible than another and perform better which means you get paid more.
I'm using in almost every page with "Wide Skyscraper (160x600)" (only for 1024X800+ resolution) but I'm also using another ad unit on the same page (for all resolutions). Well, I used for 4-5 days with the "Banner (468 x 60)" format but the text is too small and my users probably didn't see this ad very well so I today changed it to "Large Rectangle (336x280)" and until now the clicks are higher than before. I'll keep monitor it so maybe this format ("Large Rectangle (336x280)") is better than the "Banner (468 x 60)" format (more visible to my users). I also using "Banner (468 x 60)" for only in 1 page at the top (its a page with a table with 10-15 rows of content) and its fine. You need to see your website design and match to it the best format for you and your users.
Can advertisers decide if they allow their ads be displayed in adsense sites and in which format thye can be displayed?
They can opt out of being shown on adsense sites, but they can't say what format they should be displayed in.