No issues with Enterprise and Standard on the same instance. As @Magnus Ahlkvist has mentioned you don't gain anything having Standard on the same instance with Enterprise. If you are already licensed for Enterprise take advantage of the increase in available options. If you are planning on developing the new app and move it to another instance or plan to downgrade to Standard Ed for the next release of SQL then ok (good forwarded looking planning), but you are licensed for any number of instances on that hardware.
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