Is it permissible for me to use the financial aid designated for my colleagues after their financial situations have improved, or should other deserving individuals be chosen?

If some of those who receive aid are found to be undeserving, inform the donor so he may authorize you to spend it on yourself, or empower you to disburse it to whomever you deem needy. If you are needy, and the donor has authorized the disbursement to an unspecified individual or to multiple individuals, then there is no harm in taking what you need from it.