It depends on who you read. I did a PHI search for
#milia# ~ #sunt# to see if I could find any examples.
The first I came across is from Nepos' Life of Miltiades:
Hoc in tempore nulla ciuitas Atheniensibus auxilio fuit praeter Plataeenses. ea mille misit militum. itaque horum aduentu decem milia armatorum completa sunt
In that time no city gave help to the Athenians except the Plataeans. They sent a thousand soldiers. And so with their arrival there were ten thousand armed soldiers.
- NB: I don't really have a good translation for complere in military language, but it has a strong sense of presence. Rolfe translates the above as "was raised to ten thousand," for example. Lewis and Short list a number of wildly different ways to translate the idiom.
You also see it all over the place in Curtius Rufus. Just one example should suffice:
Praeterea XXX milia hominum cum VII milibus iumentorum dorso onera portantium capta sunt.
Afterward 30,000 men with 7,000 pack-animals carrying loads on their backs were captured.
It seems some authors variably have masculine or neuter. Livy e.g. has masculine at 28.28.3 (milia hominum quattuor...percussi sunt) and neuter at 29.29.3 (octo milia liberorum seruorumque...sunt capta).
So yes, there is precedent for it.