Using first name only for university email alias

You’re overthinking this issue. Nobody cares. Enjoy your cool email, and have fun in grad school.

"Could that be interpreted as inappropriate / unprofessional / overly casual by some"

I'd say yes, there is no way in general to completely prevent that. But ...

... "or would it be fine"

Basically yes to that, too. If the university has no explicit policy, I don't think anyone would take it too seriously.

To somewhat qualify this answer, I know some high-status academic professionals who signed up with a silly user name when they enrolled as basic students, just because they could (and the local custom encourages this) and are now stuck with it.

If you gain a position where you are important enough that it matters, you can probably get another alias if your [email protected] (or whatever) turns out to be harder to live with than you imagined when you originally created it.

The ultimate decision also depends on your role. If you are senior faculty, I'd say having e.g. [email protected] actually looks like a friendly and inviting, rather than an unprofessional email address. If you are not in a position where you are expected to represent the university regularly anyway, I don't think anybody will particularly care enough that this would be an issue. On the other hand, if you work in a formal role (legal? accounting?) maybe be more strict.