If it was me I would jump right in. You are already interested in SS so the conversion or at least trying it would come sooner or later. Most who are interested enough to try it get hooked.
The reason I would jump right in is that you are comming from BMX. If you choose a nice easy gear to get started you will do just fine. You will also not learn to jump on the granny any time that you get on a little hill. The other thing is climbing a SS, when you stand and really get going up a hill, the motions feel just like comming out of the gate. I have reccomended riding SS for cross training to all of my friends that still race BMX.
Just make sure to give it a little time. Do not expect to climb every hill the first time out. You might, but just don't expect it. Get in your mind to have fun and see what happens. SS can be 95% mental. If you keep thinking its too hard, then it will be. Just remember to smile!
