The message you send always has the complete text, both what you forwarded
and what you added.
The "show quoted text" is added by Gmail when the message is received by a
Gmail user who is using Gmail's online interface. The "show quoted text" is
not a link within the message itself; it is being added by Gmail in their
user interface that they (or you) are using to view a message.
It does appear that Gmail does that only if it sees that text already in the
same conversation received by that person.
On the other hand, some email readers (and this potentially could include
other online email web interfaces) recognize quoted text, usually by the ">"
or the equivalent of a "|" at the far left of each line, and may do things
with that quoted text ... including changing its color or font, or
potentially substituting a "show quoted text" link in its place (though I
don't know if I've ever seen that). That is entirely up to the individual
email program. So yes there is a possibility of this happening in another
email program or system too.
You could get around that, by not having the ">" or "|" at the start of each
line. You might need to cut-and-paste the text from the original emails
into yours, rather than just forwarding it, so that there are no visual cues
that that portion of the text was in fact forwarded.
Andy