Disabling Cross-Save does not unlink your Steam ID from your Bungie.net profile. Both your XBL ID and your Steam ID are linked to your current Bungie.net profile, so logging in with either one will still take you to the same profile.
In the Find Fireteam section, you'll just want to select Steam manually.
If, instead, you wish to unlink your Steam ID from your Bungie.net profile so that it has its own Bungie.net profile next time you log in with it, you can do so by logging in with your XBL ID then going to your Bungie.net settings under Accounts & Linking and unlinking it there.
Please keep in mind that if your Steam ID currently has any emblems granted from entering codes on Bungie.net, that it will lose access to those emblems because they're granted through being linked to your Bungie.net profile.