hey. you could defenalty swich to saltwater. just make shure to clean out everything really well with fresh water befor you start over again. im pritty shure that your filter should be fine. the more flow the better. you should be set with the supplies that you have (its all i have for my 10 gal
tank) but because yours is larger you may also want to get a small protien skimmer. you'll be happy that you made the investment later on. your also going to want to get a air pump and air stone.
once you have your tank set up with salt water, just through in some live rock ( alittle more then 1 lb per gall) and let it scycle for a month and youll be all set.
as for your last question. clown fish work well with alot of diffrent animals. they are a bit agressive so if you want to get two of them you should get them at the same time. buble anenomies are great for clownfish also becuase then you can see their symbeotic relationship. as for shrimp and crabs, peperment shrimp, and cleaner shrimp work well, and hermet crabs and emrald crabs also work well.
hope that helps you