I can't comment on the connector question, but as for renting strollers at WDW, not sure I'd do that. Unless there are other strollers there that I didn't see, most folks with Disney strollers looked as if they were having a tough time with them. Maybe okay for your three year old, but not the 7 month old. They are hard, molded plastic. Little ones would have no support in these, and they look uncomfortable for any age. Most folks were padding the heck out of them w/blankets, etc., trying to prop tired kids and keep them comfortable. I've also heard they are heavy and not easy to maneuver.
Something else to know, which I didn't until I got there, is that strollers are not allowed in lines for the rides. You have to park strollers and walk/carry children. I agree, your three year old will probably want to walk quite a bit and will do well, but you will probably want to have a Bjorn or similar carrier for your little one if you are intending to stand in line with her.