That sounds about right, although I’d suggest running local tests with a few more connections and running the same tests on Fly as well.
If your app pushes past the allowed memory, it’ll be terminated, yes. So it might make sense to run a couple of tests for an actual instance on Fly (on the actual VM, so it takes into account the OS as well) and then set your hard limit a little lower than what the app can manage.
The numbers you’re looking should be well within the default Fly limits, but if you do push against them you can start a conversation here.
We so have some ideas on paid plans coming up that let you set custom limits as well —Coming soon: paid plans and support, oh my — but in this case I doubt you’ll need that.