The seed itself has enough energy to go a good 30 days. If the hydroseed contains more fertilizer it would need to be a slow release to have any effect. When there are no grass roots in the ground, there's no need for fertilizer, so if the hydroseed fertilizer is a fast acting fertilizer, then it's wasted. Once the grass is up, you can fertilize with a starter.

You can fertilize with organic fertilizer at any time. It does not directly feed the grass. It feeds the soil microbes and they feed the grass. That process takes 3 full weeks to happen, so don't look for overnight results with organics. But organic fertilizer can go down the same day as the hydro seed goes down.

Hopefully you are talking about a future seeding event. Now is the worst time of year to seed northern grasses.

Actually, hydroseeding has a much wider window of application than does just seeding and that is due primarily to the technique. The slurry itself protects against weed development as well as protecting the seed. And the heat allows for very rapid germination, typically within 5-10 days. It is done all through summer here although the new law must be able to be well irrigated (however DOT and commercial applications seldom follow that process and seem to establish perfectly well anyway). Local hydroseed slurries typically contain a starter fertilizer but typically, fertilization is undertaken a month or so after seeding. In my area, if one were to hydroseed now, one would wait until early September for any additional fertilization.