In Welsh it looks like “to say it” is dweud e or dweud hi similar to decirlo in Spanish (I also do SSin Spanish). BUT:
Does the sort of pronoun following depend on who is saying it male or female?
What happens when the subject is “you” do we assume male ?
I’m not 100% sure what you are asking here, but I’ll say what I think I know in simple terms, and you can see if that helps or not, and ask additional questions if you like:
First, I’d better clarify that I learn/speak Gog (northern) so for me, “he” is either “o” or “fo”.
(southern equivalent being “e” or “fe”).
I’ll take a more simple example, which was used (or something very similar) in one of the northern early courses, or possibly levels:
“Dw i’n taflu hi i ffwrdd” “I throw it away” where the “it” in this case refers to a feminine object (I think the example was “pêl” - “ball”, which is feminine in Welsh).
If I’m referring to a masculine object, let’s say “ffôn” phone, then it would be:
“Dw i’n taflu fo i ffwrdd” - “I throw it away”.
As you know, Welsh has no single word for “it”, and you have to use the appropriate masculine or feminine equivalent.
I think, and I’ve never been quite certain about this, so am more than open to correction, that if you don’t know the gender of the thing you are referring to, then the convention is to use the feminine, i.e. “Dw i’n taflu hi i ffwrdd”, but as I say, I am not sure, so don’t take that as gospel.
I’ve never really thought about how I’d say “I say it” or “I am saying it”, but first instinct would be:
“Dw i’n dweud o” (although if the “rule” above is true, perhaps that should be
“Dw i’n dweud hi” ).
Now I’ve used the “long form” to keep things simple, because your question talks about the subject, and asks if the pronoun changes according to who is saying it. Well, the object pronoun will not change according to who is saying it, because who is saying it is the subject of the verb and not the object.
But when it comes to so called “short-form” verbs, the “verb-subject-object” order of Welsh becomes more apparent. (It’s there too in long-form, but it’s a bit disguised by there being an auxiliary verb).
I will use the simple past, because that’s perhaps the more likely time you will use short form for this verb, and again, this is the northern version, and the southern one looks slightly different:
“ddeudes i” - “I said”
“ddeudes i rhywbeth” - “I said something”.
“ddeudes i hi” - “I said it”.
[“ddeudes i fo” - “I said it” if we were using the masculine pronoun for “it” ]
(In more formal writing you will see “ddweudais i” or maybe “dywedais i”.
I can’t remember how SSiW would write it in the notes, but the pronunciation I learned (and actually what people in the north seem to say) would be written something like:
“ddudes i”
…but sorry, that’s a slight diversion from your question.
Anyway, to try to summarise, the short answer to your question is that the subject of the “saying” verb should not affect the gender of the “it” word either way - it would only depend on what the “it” refers to and if it’s indeterminate, which I think “say it” usually is, then I think the “it” is “hi” in this case. (But, that’s open to being corrected! 
).
Looking from a slightly different angle. As I recall, In the challenges dweud e/hi means “he/she says” as part of a sentence. So the he/ she comes after the verb. A bit like " says he". So yes, you have a choice of the Welsh words for he/she/you (chi or ti)/we/they.
I hope this helps.
PS. For It, see above
In Welsh fo/fe/hi mean he/she but also it. Every it is either masculine or feminine.
I would suggest “Dywedodd hi amdano fo” =She said about it [where the it is masculine]
to translate ‘to say it’ would be 'deud fo/hi’as in Fydd i’n deud hi nes ymlaen = I will say it later
You is di/chi, it makes no difference if the you is male,female or anything else as far as I’m aware.
Remember that how Welsh works will not map perfectly with Spanish as it doesn’t with English.