I have no problem mixing fluorescent tubes with LED tubes. I would remove the ballast and install some direct-wire LEDs in the existing fixture, and leave the other fixtures alone for now. Save the fluorescent tubes if they're still good and use them in the other fixtures. Once all of your spare fluorescent tubes are used up, start replacing them with LEDs as they burn out. You can buy hybrid LED tubes that can work both with or without a ballast, so you can mix LED with fluorescent in the same fixture and upgrade one tube at a time as they burn out. Then when the fixture is 100% LED and/or a ballast fails, you can remove the ballast. That's exactly what I did in my garage - I have one fixture that's all LED- the ballast failed, I removed it and direct-wired the LEDs. The other fixture is half fluorescent, half LED, and of course still has the ballast installed. The last fluorescent tubes will be replaced with LED as soon as they burn out, or when the ballast fails, whichever comes first.