The flox help says:
flox <development-command> \
[ --stability (stable|staging|unstable) ] \
[ (-d|--date) <date_string> ] [ <args> ]
but when I type flox develop --stability unstable I get
error: unrecognised flag '--stability'
(I’m using flox-0.0.6-rc45).
Thank you for the bug report. It is with the engineers and we hope to have an update pushed out later this week. 
I’ve fixed that part of the usage string to read as follows:
development commands:
flox [--stability (stable|staging|unstable)] \
[(-d|--date) <date_string>] <command> [<args>]
----
flox init - initialize flox expressions for current project
flox build - build package from current project
flox develop - launch development shell for current project
flox publish [--publish-to <gitURL>] [--upstream-url <gitURL>] \
[--copy-to <nixURI>] [--copy-from <nixURI>] \
[--render-path <dir>] [--key-file <file>]
- build and publish project to flox channel
flox run - run app from current project
flox shell - run a shell in which the current project is available
You should find that flox --stability unstable develop doesn’t throw an error. The updated version should hit production some time tomorrow.
Of course there is still the related issue that the --stability flag isn’t actually changing the stability used for the subsequent develop invocation, but we’re working on that too and hope to have a fix out for that by Tuesday or Wednesday.
Thanks for the bug report!
Thanks Michael for your prompt reply and upcoming fix 
Just wrapping up thread and confirming that flox has been updated and the current version is flox-0.0.6-r49. Any issues, just let us know. 