I have some questions:

Do I need to create custom voiceType for a falmer unique NPC? I created custom race, where I checked allowing to speak, added script on that NPC that would allow to speak, still sometimes it does speak, sometimes it doesn't.

And is there some way, how to check what could be that problem? Like if there is a prompt for speaking, but he says nothing, then it would be a problem in quest stages, but if there is no prompt - I have wrong something with the creature?

And in which cases I should create unique voiceType? For a group of NPCs or if some unique NPC comments a lot? and do I have to create idle comments, because it will not inherit anything?

And if I would create that custom voiceType, I wouldn't need to generate SEQ, or is it still needed?

Because Creation Kit tutorial says nothing about it, only some other tutorials use custom voice type for custom followers, and in the main file of Skyrim the quests in Skyrim use custom voice type for dialogue quests for repeating sentences, but sometimes for unique NPCs - so is it for events or do I need it for an ordinary unique creature NPCs where I can use quest idles?

And for Falmers - I need their default race idles for combat, so I guess I shouldn't create custom voice type if I want to keep it?

AllowPCDialogue() runs on the reference, allowing the player to speak to that actor, even if the actor's race isn't flagged as allowing PC dialogue. If the race allows PC dialogue, then you don't need to use a script. If you want your custom Falmer follower to be considered a Falmer by other functions (for example, Serana in Dawnguard has some combat lines against Falmer, and if your Falmer follower died, you'd probably want the player to be able to harvest its blood for "Discerning the Transmundane"), you should probably have it be FalmerRace but attach a script that will enable the Falmer to speak.

If no prompt to speak shows up, the actor doesn't allow PC dialogue. If the prompt shows up, but the actor doesn't say anything, the actor doesn't have any valid hellos. The Falmer voice type isn't used by any NPCs that speak, so Falmer probably don't have any hellos. You can create some custom ones for your follower to use. If you'd like the Falmer to speak -- that is, say words -- you could record audio. If you'd like the Falmer to behave similarly to a dog follower, you could simply have the hellos and other dialogue be written as "(growls)" or something similar, use a blank .fuz file, and use the sound section on the info to play an appropriate Falmer noise.

Unique versus generic voice types are purely a matter of personal preference. If Bethesda had wanted all NPCs with unique voices to use generic voices, it could have done so. It's just for variety. NPCs with unique voices generally don't have have dialogue outside of their quest-related stuff -- presumably because someone like Ulfric doesn't need dialogue for, say, participating in a brawl.