i don't know much about hydraulics but there has been only been a couple of featherweights that have managed to incorporate hydraulics and at great cost. you'll probably need to get your pump and maybe ram machined out of aluminium as most pumps are made from cast iron and weigh a ton!

yes you can use a double acting cylinder as a single acting, i wouldn't bother using a spring and just use a bungee cord to return flipper arm.