you forgot one more question and that is which type of milling. climb milling or conventional milling.
1. what are the best endmill i can use for foam milling ? ball, ..etc
well i would start with a 2 flute centercut for plunging. foam is plastic and is subject to heat. if you have any heat build up you will ruin the finish. the thing i would look at is using a HIGH VOLUME OF AIR to keep any chips from sticking to the endmill and clogging up the flutes. once the flutes are clogged you'll only be burning the foam. as for ball end or square end that will depend on what you're machining i.e. curved surface or flat or 3D. this will also dictate what size. you'll want the biggest endmill that you can use but your geometry will tell you the actual size. as for type of milling climb or conventional your material will tell you. make a few cuts to see how the foam comes off. does it leave a clean surface or is it raggity. so then you can program around that.
2. how deep should go in each path ?
don't be tempted to go too fast and too deep. use the same basic rules as you would for metal. go less than 1/2 the diameter for side milling and less than the diameter deep.
3. what speed should i use ? lower or higher is better ?
4. what travel speed it better ?
these 2 questions should be qrouped together. based on load per tooth and the rpm you will dictate what the feed rate is. rpm is judgement. take those test cuts and visually decide. too fast will burn too slow may rip so you need to do some test cuts. the foam material i'm assuming is not a tuff cut. so you can likely make a fairly heavy feed rate. i know that when i cut a solid plastic i run up to 3000 rpm with a 2 flute maybe somewhere between 10 to 20 ipm. just depends how much i have to do. these speeds are more for a smaller end mill say under 5/16" [8mm].
do your tests then decide. you might also check out the niagara endmill site. they may have a chart on your material.