I have flown my versa *mildly* tail heavy and my SEMFF Stingray strongly tail heavy, and while it felt like they flew like hurled feces, it was controled flight, complete with aproach and landing. By NO MEANS a fun flight, but controled.
Do get the CG as close to the recommended neutral ballance as possible -- every bit of nose heavy will cost you in elevator authority, every bit of tail heavy will cost you in fighting the elevator trim as speed changes.
Also, if it immediatly nosed in without accepting control input . . . are you sure? If you haven't, check the directions of the control surfaces -- The feedback between your hand/eye and the plane's response for a reversed elevator is AMAZINGLY fast, and can look like it ignored your input and took a b-line for a dirt-nap.
edit: ok, re-read the post . . . do check the surface directions, but . . . give it back pressure if it needs it on launch. Part of trimming the airframe is finding where "neutral" controls are for all the imperfections in the airframe. Havign a little bit of reflex (intentional up-elevator) on a wing is very common.