I would like to know if there is a way to compare the relationship between a factor (length) and a response variable (lipid content) among multiple groups? I have given up on ANCOVA, as length is continuous. I am unsure if a multiple regression allows the use of a categorical variable (group membership) as an IV.
As @PeterFlom noted, multiple regression and AN(C)OVA are specific versions of the general linear model. ANCOVA's specific difference from ANOVA is that it permits modeling continuous covariates like your length factor. ANCOVA's difference from multiple regression is that it suits categorical factors like your group membership variable. ANCOVA would probably work just fine for you, but multiple regression will work too. (After all, they are the same general linear model!) You'll just need to code group membership by one of many schemes suited to multiple regression if you decide to go that route, and figure out how to interpret the results in this somewhat different format.