From playing the guitar (for fun), I would always try to make a chord leaving my pinky free to either add flavor to the chord or play other melodic "riff" notes.
Question - Would I be doing myself a disservice if I play triads on piano with my 1-2-4 fingers to again be able to add extra notes with my now-free pinky?
1-2-4 is fine; you won't do yourself any disservice.
In fact, 1-2-3 is often used for exactly the reason mentioned in the question: it frees up 4 and 5 to add additional higher notes. Sometimes there might be an added lower note, and the basic triad would be played 2-3-5 (for example) so the thumb can take the lower pitch.
1-3-5 is the standard way the chords are taught, because it has advantages for other aspects of teaching, such as giving a consistent fingering between right and left hands (RH: 1-3-5; LH: 5-3-1) and showing the relationship between chords and (5-note) scales. But in practice, fingering depends on context.
You’d often be required to play a triad 1-2-4 if you for example have a subdominant progression with the subdominant in second inversion (e.g. C-E-G as 1-2-4, C-F-A as 1-3-5).
In piano playing the fingering is chosen very carefully to allow you to get smoothly from one point to the other. A skilled pianist should be able to shape the weight of a chord however he wants without depending on specific fingers. As a beginner pianist you should definitely learn by the book, so that you learn the technique and develop control.
By the way there is quite a lot of piano literature that plays a melodic line on top of harmony in one hand. My piano teacher liked to say: "The most beautiful melodies are written for your right pinky".
In addition to the other good answers, keep in mind that the piano has a sustain pedal.
On guitar, sustaining a note requires* keeping your fingering. So, adding a riff requires having available fingers that aren't being used for the chord.
However, piano frequently takes advantage of the sustain pedal, allowing you to use up to all 10 fingers to add a riff to a chord, and the original notes can still be sounding. Going further, you could have dozens of notes involved in a single chord by keeping the sustain pedal down and making your way across the keyboard.
I would recommend learning piano fingering as they are traditionally taught.
*As Tim noted in the comments, there are ways to sustain notes on guitar that don't involve keeping your fingering. Certain pedals, harmonics, and other devices/techniques allow for riffs to be played along with a chord.
The fingering for arpeggios is just for that - arpeggios and block chords. Whichever fingers get used will depend, to an extent, on who's playing them, due to hand size, dexterity, etc.
In order to play the melody over chords, using the same hand, will, of course, necessitate having a free finger (or two) to do just that, and it makes sense that it's pinky (and sometimes ring) that come into play there.
So, the two things are slightly divorced from each other, as each has a separate function. But it's the usual way we add 6ths, 7ths etc., to standard shaped chords anyway - it's what guitarists have always done. (Just comparing the same situation with another instrument).
When playing a melody, the highest notes are usually the ones perceived to be that melody, and since pinky is situated there, that's what gets used. It's often the weakest finger, so extra practice will be needed to make that melody sing out, but it's worth the effort. It will also, at times, be expected to stretch further than its usual distance when playing arpeggios or 'normal' chords, so examining what notes can be played underneath by the remaining fingers is paramount. Keep going with it!
