@CoolGuy and @ForceMel gave two good points. Write what and for what you want, and explain to newbies that their Level 1 troops will be upgraded to the recipient's troop level, if the recipient has that same troop at a higher level. Many do not know this and so are afraid to donate to higher level players.
I've been to dozens of squads, and most had donation rules. The most bizarre thing I saw was not to use donations to attack, just for defense. If you asked for troops again, you would have to share the replay to prove that you lost the troops defending and not attacking. Risk of being removed.
A good rule is call your donation, say what and to whom you will donate, before donating to prevent your mate from receiving the duplicate unit in case someone asks for different units.
Establish what cannot be donated (droids, dewbacks, banthas, and so on) unless it was asked for some purpose such as deploying banthas/dewbacks, for example, and make sure everyone understands it.
Establish a simple pattern. Attack or defense? And what troops can be donated, so write "Atack" or "Defense" in the request.
If there is nothing but "Please send troops", then don't complain.
Many squads make donations a workhorse and I've seen several members/officers turn out to be true drama queens just because they received something they didn't ask for. If your squad is open to all, it is no use complaining because you has received 4 soldiers or banthas. Restrict access and interrogate recruits if they understood the donation rules before accepting them.
And I've seen officers who didn't like the member and sent the wrong troops on purpose. I've been through this myself, so instead of complaining, I only quit squad.
SWC Windows survivor.
Imperial HQ 11 & Rebel HQ 11 on vacation.