You should consider getting more neon tetras probably a shoal of 10 ~ 12, the more numbers the more secure they feel and the happier they'll be. Barbs, obviously not, aggressive fish like Bettas. . .
With chinese algae eater, I've read that these things hook themselves to slow moving fish, so your Betta might not like him so much. Along with that be sure he has enough algae, or he will most definitely eat your Betta. So it's a fifty fifty with the algae eater, you could try some small otos if you're worried about the algae eater, they won't bother your Betta.
As for other fish, you could try bottom dwellers like cories, otos, any fish that is not aggresive and have no long finnage, would do fine with your Betta. Although it really depends on your Betta's personality and if he's docile or not.
But other than that your Betta would enjoy the grand space of a 30 gallon! Post pictures and keep us updated.