So, i was in spectator mode in a ocean temple, but i could only see the elders, the guardians did not show up

Confirmed.

I think this is intended. Since you're in spectator mode, you're not allowing any mobs to spawn in your region (like they would in survival/creative/adventure). Elder guardians, however, are boss mobs, and therefore stay around anyway.

Well, elders are not yet considered bosses yet, and other mobs spawn in spectator

Last time I checked, mobs don't spawn in spectator mode because they require a player to be in range, and spectator mode does everything it can to make the player seem like they're not there, so mobs don't spawn. Have you ever tried testing a mob farm in spectator mode? Doesn't work. At least not for me, anyway. And I believe the elders are there because they spawn in the temple and don't despawn.

elder gaurdians don't despawn