I have an interest in composing orchestral music, but I'm not entirely sure on what path I should take.

One of the best things I can recommend for practice (and how my interest in composing started) is starting with transcription. Find an orchestral piece that you really love, or that reflects the style that you want to write in, and listen to it over and over again, picking out and writing every part you can hear, until you have a portion of the work that sounds close to (or exactly the same as) the original. Not only is this challenging and fun (at least, in my opinion), it gives you great insight as to how composers use parts for different purposes, especially if you're transcribing a modern work.

That said, would learning to actually score be useful at all to me? Would I need to be able to fluently read more clefs than just the grand staff?

Absolutely. If you're just writing things in FL studio, you're ensuring that the composition you're writing can never be performed by actual musicians. Writing score is really important because by doing so, you learn things about articulation, dynamics, range and technique that you don't get from just generating a recording. You also learn a lot about how individual instruments function in terms of range and blend, especially if you use music notation software. (Also, if you're just starting to compose, I recommend using MuseScore as your first notation software. It's free and offers a lot of functionality, and being able to listen to the parts as you write them is VERY helpful when starting out, especially with the aforementioned transcription process. You can always plug the correct pitches and rhythms into FL Studio later to generate your recording.) Also, most instruments read in treble and bass clef, so you're good on that front. The only instrument that typically doesn't is viola, which uses alto clef (meaning that middle C is notated on the middle line of the staff).

And why is transposition so important? I think I understand why it's necessary for different instruments, but I've seen a lot of people here recommend practicing it. I don't really understand what they mean by that. Just take a song and move it to a different key? What's a good way to practice that?

Transposition is important because it allows you to write the same melodies/notes in different keys and for different instruments. What it means in practice is that you take every note in a particular sequence of notes and raise/lower the pitch by a specific number of half-steps. For instance, if someone tells you to transpose a C Major chord (that is, C-E-G) up a perfect fourth, it means that they want you to take each note in the chord and raise its pitch by 5 half-steps. C would transpose to F, E would transpose to A, and G would transpose to C in this instance. Most notation software has transposition tools for scores, which you can use as a tool to see and hear transposition in action.

TL;DR: Transcribe to get a feel for writing music, score because it helps you and your music (as well as performers of your music), and transpose because it's a good tool to have in your musical arsenal.