I'd tactfully find a way to tell you to pound sand if I was your Soldier and this came across to me. It isn't in a squad leader's discretion to tell a Soldier on how to spend their annual clothing allowance.
You were in the wrong to tell all of your soldiers to buy new uniforms. Like others have said, you overstepped your bounds
I'm going to devil's advocate this situation:
I think you were well within your rights to inform your Soldiers that their uniforms were out of standard and that they needed to get new ones. I myself have told Soldiers numerous times their uniforms were sub-par and they needed to update their gear. If my Soldiers uniforms look like ass, I let them know and I expect the situation to be fixed promptly.
I myself have been corrected and "advised" to get a new set of ACU's or to spruce my boots up. This might be different because I work in Joint environment with normal everyday interactions involving 0-6's and above. I expect myself, and the limited number of Soldiers here to look their best at all times.
Your tact in the way the message was delivered might have been flawed, and your timing could have been better (should have given them until at least after a pay period to get the items). Additionally you should have told the Soldiers that if they have issues (financially) with getting new uniforms then they could talk to you off-line. At this point budget classes (through AER etc), and financial responsibility counseling should be done.
Lastly, telling your NCO to “pound sand” because you didn’t like his order to get new uniforms is childish. The order was not illegal nor was it immoral. I understand you might not like it, I understand that you might think he overstepped his boundaries, and I understand that its “your money, and you can do what you want with it”, but the bottom line is the SM needed new gear, the NCO directed the gear get purchased, and telling him to pound sand (or in this case telling ME to pound sand) will get you about as far as an airplane without wings or a motor.