I’m in love with the boy names [name_u]Seth[/name_u] and [name_u]Kai[/name_u]. The only problem is that our LN is one syllable (starts with N). I’m just wondering if that’s a major no-no. The middle names we’re planning on giving are all two- or three-syllable, and I know that that helps the flow a bit, but it’s not as if they’re going to go around introducing themselves with first and last names. It breaks my heart to have to let these names go, but I’m wondering if it matters as much as I’m thinking it does.
I don’t think it’s a no. Sometimes one syllable last names and one syllable first names do sound ok, especially if they aren’t word names.
In the end, it’s up to you. I would hate to think of you not using names you really love because you were worried about something as trivial as syllables.
There are on syllable first and names that work fine together. I see no problems
I see no problems with names like
[name_m]John[/name_m] [name_m]Smith[/name_m]
[name_u]James[/name_u] [name_u]Dean[/name_u]
[name_m]Clark[/name_m] [name_m]Kent[/name_m]
[name_m]Bruce[/name_m] [name_u]Lee[/name_u]
I think [name_u]KaI[/name_u] would work particularly well with a one syllable last name beginning with N.
It is definitely not unheard of:
[name_m]John[/name_m] Dear
Artists that do not go by their longer first names:
[name_m]Tom[/name_m] [name_f]Cruise[/name_f]
[name_u]Chris[/name_u] [name_m]Pine[/name_m]
Others:
[name_u]James[/name_u] [name_m]Bond[/name_m] 
As long as you vary the sound of first and last name, it can work beautifully. Good luck!
I don’t think it’s a bad thing. I think it depends on how they sound together. [name_m]Just[/name_m] as an example of a 1s/1s name: [name_u]Seth[/name_u] [name_m]Bach[/name_m] sounds fine.
The “rules” are silly. [name_m]Say[/name_m] the first and last name together. [name_f]Do[/name_f] they sound okay to you? Then it’s fine.
I think one syllable names tend to really work with one syllable last names - particularly with men’s names. I immediately thought of some celebrity examples, and I see other posters had the same idea. I think it makes the names memorable. Its catchy and causes people to remember the first and last name together. Like [name_m]Tom[/name_m] Hanks or [name_m]Tom[/name_m] [name_f]Cruise[/name_f] or [name_u]Jude[/name_u] [name_m]Law[/name_m]. [name_u]Seth[/name_u] [name_m]Nash[/name_m] for instance, sound great to me. I think [name_u]Kai[/name_u] is a little trickier because it could be hard to tell where one name ends and the other begins. [name_u]Kai[/name_u] [name_m]Nash[/name_m] = Kine [name_u]Ash[/name_u], potentially. Good luck!