If the total income after subtracting credits equals a negative number on IT-150 - do I need to continue filling out the form? Am I entitled to a refund?

You should indeed continue with the form if you want a refund, which it certainly sounds like you are due. Leave lines 25 and 26 blank (since line 24 is more than line 21), put $0 for your NY State taxes on line 27, show your taxes withheld on line 47, and your refund on lines 51, 52 and 53 as appropriate.

Thank you!