Although it says 2500 hours on the website Monarch view that as an ideal rather than a minimum. In most courses including mine experience has ranged from 1000TT to 6000TT.
They also take cadets with 200-250 hours from CTC. So with 1500 mostly on the Dash 8 you'll be fine. Put in an application - they are always short of people in MAN.
Incidentally the bloke with 1000TT I mention above was also on the Dash 8 prior to Monarch.