Whether itc can be availed against cst sales?

Yes, but after adjusting input tax credit against the output tax payable under the VAT if there is any balance of tax credit, such credit shall be adjusted against the CST payable

Now suppose,

Input Tax is Rs.10000,
Output Tax is Rs.4500,
and CST Sales Tax collected is Rs.5000, then, entire amount of Rs.5000 can be availed as ITC right?

And there is a Rs.500 to be carried forward right?

Yes it can be carried forwarded to the end of the next year. If there is any excess unadjusted credit in the end of the second year then you can ask for refund. this differs from state to state.

Check "CARRYING OVER OF TAX CREDIT" for detailed explanation:

https://www.icai.org/resource_file/18948sm_finalnew_idtl_service_cp8.pdf