Most startup founders do not have a separation of business and personal life or have very little separation. When setup properly outgoing emails will be sent from the correct address. There are no security concerns, someone can easily download all their mail if they're trying to get a copy.
It depends, if they're starting a company then they should be setup separately from the start. You can't honestly tell me that you want Frank from the call center at your health provider getting a copy of your health records that you just sent in to his personal Gmail account?
If they're simply creating something to make some extra money then sure you could use the same account, but to be taken professionally you should keep them separate.
