If I enchant my boots with this stone, can I also do my helmet and have 250% feather?

You enchant anything you wear with them and get the CE effect. If I remember correctly the Feather enchantments will stack so yes, if you have enough slots or powerful enough stones you can reach 250 feather.

Yes, you'll have 250 feather. Although if your character is good at alteration, there are some very good feather spells that have long durations. If you can, use the spells instead and save your enchantment slots for other things, if you wish.