Apart from differences in pronunciation between the various parts of Wales, and that there are about five main dialects, not two... no.
You may have bumped into one or both of two things, perhaps:
Firstly, adjectives mutate after feminine nouns anyway:
- car glas, het las - a blue car, a blue hat
- blwch gwag, ystafell wag - an empty box, an empty room
- ci dof, cath ddof - a tame dog, a tame cat
Secondly, some adjectives have separate masculine and feminine forms, usually with a change to the vowel in the middle of the word. Nowadays the masculine form is acceptable for both genders, but some people like to use the different forms if they know them. Remember that the mutation of the initial letter happens as well as the internal vowel change:
- car melyn, het felen - a yellow car, a yellow hat
- diwrnod byr, stori fer - a short day, a short story (stori fer is very common)
- llyfr gwyrdd, coeden werdd - a green book, a green tree
- bwrdd trwm, cadair drom - a heavy table, a heavy chair
- car gwyn, het wen - a white car, a white hat
A few adjectives have different plural forms, too.
In the notes it gives examples of feminine forms of adjectives, eg gwyn/gwen, and also of mutations after feminine nouns. One example is: ci gwyn, cath wyn. This appears to be using the masc adjective and mutating it when used after cath.
The next example is: y ci gwyn, y gath wen This appears to be the fem adjective mutated, and throwing in a mutation of cath for good measure.
It would be helpful to have a list of common feminine nouns. I looked up dillad for a question about red clothes and it appeared in a list of feminine nouns so I put dillad goch and was told it was a typo and should have been dillad coch.........
About a third of nouns have feminine grammatical gender, so a list of even the common ones would be hard to manage. Any Welsh dictionary will indicate the gender and the plural. It is probably best to learn genders and plurals as you learn the nouns.
dillad is a plural, so it has no gender. Use the masculine forms of adjectives with plurals - some adjectives have plural forms, but they are less and less used these days in the colloquial language.