the only word is Genesis
There are three 7-letter words possible:roosterrooterstoreros
Can't get a 7 letter word with only 6 letters. The 6 letter word is DESIST.
Stretch is a 7 letter word with e as the only vowel. Twelfth is another 7 letter words containing only one vowel.
The only 7 letter word for cane I can think of is support.
Breeder is a 7 letter words using only those letters
There are no 7-letter words with those letters, there are a few 6-letter words though:grieveregivereviewviewerweever
Hipster
Respite.
fetuses
Saguaro
There is no 8-letter word that can be made, there are these 7 and 6-letter words though:senegasagenesagonesgenoasgnosesgossanseasonsegnossenega