My Platoon Sgt has ordered us to get name, rank, and a reflective E for eco troop sewn on ruck and FLC at our own expense. Can he force this?

Yes. Stop complaining and do as your told.

Generation Y (why?) in action here. Is it worth the small amount of cash to skyline yourself? You can fight it and end up paying for it as long as the PSG is in charge of you. Lots of weekend duty and special details could be in your future. Pay me now or pay me later. Think about it.

As others have said, just do it. I worry about your future in the Army if this is the kind of thing that you're questioning. You make more than enough to pay for a few alterations to your gear and if it's something you can't afford, then you need some financial counseling.

I certainly wouldn't want to go to the mat with my CoC over 25 bucks. Pick your battles, PFC.

I'm not sure this applies, but in the Marine Corps, if you permanently modified or personalized any 782 issue item, you just bought it And you cannot use it while on duty.

To answer your question, no. He does not have the authority to order you to buy extra gear. If it is required for service, it should be issued. But this is not a really regulation/non-regulation issue.
Let's be real here -- if the PL wants everybody in to mark their gear in the same way it is a really good idea for you to comply. Notwithstanding the sheer practicality of it -- and, trust me, his idea is a good one -- bucking him on this will not go well for you. People are people and non-conformists are not welcome in the army. If you don't want to buy into the group think then you're setting yourself up as a non-team player.
Okay - If it's a money issue (and it nearly always is) talk it over with him and see what can be done.
But if it's a matter of principle then he is likely to ask himself why do you not want to conform to the unit ethos. Are you going to be a discipline problem or a disruptive element in the unit?
I can guarantee that most of your NCO's will be thinking along these lines. Yeah - he cannot order you to buy the gear and you are perfectly within your rights to refuse.
But there's a huge world of difference between being "right" and being welcome. Are you prepared to be shunted to the side? Passed over for promotion? Assigned more rubbish details than your buddies? Are you ready to pull more than your "fair share" of CQ, KP, casual detail? That's where this will go. If your PL and PSG have are least half as savvy as most leaders are, they will dress it up in ways that cannot be fought.
It's as simple as this -- nobody likes a barracks lawyer. Suck it up and buy the stuff already.

Why would the PltSgt make this demand of his troopers? He has the blessing of the CoC? He/she must feel there is a good and proper military reason for such a move. Unfortunately it is not something I would fall on my sword against doing. Learn from the individual's example and remember this in future years when the shoe is on your foot.