Just opened an SRS account. Can I only invest my funds in DBS by opening a DBS vickers account?

Dollar for dollar it will offset your Tax. You should top up to the amount that allows you to move a tax bracket.

you can invest with any platform just provide that platform with your SRS number

I top up to the yearly cap.

As long as the sums deposited into SRS, they are tax relief.

Once you have a DBS vickers account, you need to link your brokerage account to the SRS

The fees and charges are the same regardless of investing in cash or SRS account. The lowest commission to pay out is if you invest $5,555.00 so if you have only $500, just throw into SSB. Wait for it to be higher before investing in stocks, unless you expect the stock to moon.