Maybe serving in the British Army gives him a UK passport and that gets round the rules and regs.
No idea but could be the reason otherwise like you it seems odd especially if he impresses in the trial games

Ah ok, yes possibly. Unless he ends up signing for Donny this year with a view to joining us for 2020. We'll see.

Isn't there a cap/quota dispensation for players coming from Union?

I don't think being in the British army gets him a uk passport, but Fiji's commonwealth links allow him to be in the army.

And any service overseas is counted in uk residency qualifications.

But he would still be classed as non fed trained and we don't have any of those spots left.

There is cap dispensation. Not sure on quota... One to watch I guess.

Anyone given "New Talent Pool" status, can apply to be granted "Federation Trained" status.
Not guaranteed to be successful though.

he is too good to play the season with Doncaster
I wouldnt be surprised to see him in a first team shirt for this coming season.he has the build for Rugby league,strong runner and we know he has pace.

Radford has suggested in the HDM that he might not be just a reserve player. Interesting comment. I suppose he could mean him playing for Doncaster.

Has anyone have any idea how old Naulago is?

I reckon the plan for Naulago is for him to learn the game playing reserve grade this year with possibly a spell at Doncaster. If he shows real developement than he may earn himself a first team contract for 2020 and beyond. But he ain't getting one at the moment based on a couple of months training and a friendly run out.