Latin question

I am skipping way ahead in Wheelock for this bit of grammar, so I am probably getting it all wrong. If I wanted to say "watchdog", I would do something like this:

genitive of dog: canis -> canium (gen plural)
nominative of watcher: custos -> custodes (nom plural)
watch dog = canis custos (sing), canium custodes (plural)

Or I am out of my mind and you, dear reader, will kindly explain to me that I have misunderstood.

Man, I should just find a class somewhere and take it.
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According to my old school latin vocabulary watchdog is transated as "canis catenarius" or "canis vigilans", both nom singular.
"canes catenarii" or "canes vigilantes".
Since catenarius means chained up, I would use vigilans instead.
Hope I was useful.
Domo domo!

Though I think I am looking for the connotations of "custos" in the particular phrase I'm trying to construct, which is a name for a Super Sekrit organization within the Watcher's Council. Watcher's watchdogs would be sort of the phrase I'm looking for. Which I guess would then be nominative watchdog genitive watcher.

canis vigilans custodis

Which is kinda clunky. Groan.

ratin-go wa muzukashii desu yo. Seriously, I find Japanese easier. It's the highly-inflected thing. Tears hair out; goes and does something else.
Maybe you should tell me the exact name of the organization, so I could try to translate it.
The problem with latin though, is that it wasn't a simple language like english, so most likely the resulting name won't be an impressive one.
Grammar obsession good. I read a web page all about this that seemed backwards to me, and turns out it is backwards. Grunt.

I think the sad truth is that I just need to keep slogging through slowly and not try to get cute.
I hope I don't make you sorry you offered :) I keep wanting my magical rituals to be in Latin in the novel-length bond story. More churchy that way, you know?