Any Latin experts out there? Would (could) a feminine version of homunculus be femininculix?
Nope.
I know that homo becomes homun (I don't know why);
suffixes such as the diminutive suffix
culus, M
cula, F
culum N often modify the previous vowel and frequently but not always assimilate or modify the preceding noun as well.
homo in the nominative becomes
hominis for the genitive.
does femina therefore become feminin?
No. But homo can be feminine as well as masculine and can refer to a female human being as well as a male human being. It can also refer to humanity as a generality. It's theoretically possible to have
homuncula as a feminine nominative singular form of
homunculus.
Also, if -culus pluralises to -culi, how does the feminine -ix suffix pluralise?
ix isn't a feminine counterpart of the
us ending. It's a feminine counterpart of the
or ending.
ix pluralizes to
ices for the nominative and accusative,
icum for genitive and
icibus for dative and ablative.
But, none of that is relevant to your main question because the
ix ending is third declension feminine. As soon as you put the ulus, ula, ulum suffix on a noun, it automatically becomes first or second declension. In the case of feminine, definitely first declension.
Singular
Nom homuncula
Gen homunculae
Dat homunculae
Acc homunculam
Abl homuncula
Plural
homunculae
homuncularum
homunculis
homunculas
homunculis
just for fun,
homo meanwhile, does this....
homo hominis homini hominem homine
homines hominum hominibus homines hominibus
You've gotta love the third declension!
expert and teacher of Latin. 
Never mind and carry on.