if i got a school boy rugby cap for one country would i be able to play senior rugby for another (england u16 and wales senior)???

Yes. If you've lived in Wales for 3 years, or have A Welsh grandparent or parent, or you were born in Wales, you can play for Wales. The fact that you played for England at U16 level doesn't bar you. Plenty of senior internationals have played for other countries at junior level.