ok, no one answered me here but i came to some understanding about my questions, here you go :
1- LVM is good if you think you might be short on space on 1-2 of your important partitions. for example if your /home does not have enough space, LVM helps you to add space too it far more easier than the time you didn't made your partitions without LVM . it's something like have a dynamic partition (or something like that but not that simple!)
2- each of the partitions are separate and the / will not include other partitions if you create them separately even if you use LVM.
3- for partitions bigger than 6GB you'd better use ext3 filesystem.
4- i'm still thinking about it!