Is this going to cause a crash at some point?

[Redshank]

So I'm safe then?

Probably not. I've had ctds with this kind of thing a couple of times. The one occurrence I can remember was when playing as Pontos I tried to artificially bolster the garrison at Chersonesos in 263 to ensure the Sauromatae didn't get their sticky mitts on it before I could complete my invasion force for the following year.

Upon actually getting there when I assaulted the city I got a ctd because simlarly the units I'd created hadn't shown up properly on the strat map beforehand - and that kind of thing isn't a one off in my experience. So, personally I'd recommend you either auto resolve if necessary or reload and when using 'create_unit', if you're trying to add cavalry, I'd add some Cappadocian Medium Cav or the Generic Asiatic cavalry instead.