I racked it to another 6.5 gallon plastic bucket as a secondary fermenter but then I found out that a glass carboy is best because it has less headspace & less oxygen in contact with the beer, Any help is greatly appreciated!
5 gallons of beer in a 6.5 gallon bucket has the same volume of headspace (1.5) gal, as 5 gallons of beer in a 6.5 gallon carboy. the surface area may be a little different based on the configuration but this is unlikely to be a significant change over several weeks of fermentation.
That's why you would never use a 6.5 gallon carboy as a secondary. 5 gallon carboy only and limit head space, unless you are replacing the volume with more fermentables such as fruit. Also, if you are to do a secondary in a carboy you want to transfer while there is still some Co2 in solution or introduce a secondary fermenation with fruit, bugs, etc.
That said, at least with a glass carboy you can seal the secondary. You can;t really seal a bucket no matter how hard you try. It is always 02 permeable. Not only that, but fruit flies can and will find a way inside a bucket. The bucket seal won't stop them. Apparently it is the larvae that can get in under the bucket lid.