SEP-IRA contributions are separate from regular personal traditional IRA contributions. If you are making both, they should be made to separate accounts to avoid the custodian confusing SEP contributions with regular personal contributions; your particular custodian might ever require separate accounts. By default, contributions to a SEP-IRA are classified as SEP contributions and contributions to a regular personal IRA (not a SEP or SIMPLE IRA) are classified as regular IRA contributions.
Yes you can use the same SEP-IRA for all of your SEP contributions.
If not already established, the deadline for establishing and contributing to a SEP plan is the due date of your tax return.
