nems really need a stable tank. Inverts are very sensitive to changes in water parameters.
A general rule of thumb is wait a year. I think that's partly tank maturity and reefer maturity
For example, someone who's very experienced in reef keeping could set up a tank and add a nem in a month and be successful, as in not kill it. This is because they've all ready made all the mistakes in the past and can get a system up and running and stable much quicker than someone new to the hobby. Is it best for the nem? Does it thrive or just survive? dunno...
A pretty forgiving nem is a bubble tip. If you do decide to house a nem. Be sure to read up on them first. They do have very specific care reuirements from where they need to be placed in the tank (rocks vs. sand) to lighting requirements.
Nems also wander. So it can get sucked in to a powerhead, shredded, and nuke your tank. So there's a risk/reward factor too. Oh, and depending on the species, eat fish.
I happen to have a huge rose BTA in a 180. It's roughly 18" x 14". It was this size of a dime when I got it, lol. I'm not trying to dissuade you from nems, they're awesome. Just be sure to learn about them first. I've seen a lot of sad stories and people get discouraged.