I have a Handi Rifle that came with two barrels, a .223 and .308. The .308 is a perfect fit on the frame, the .223 has a bit of play when the foregrip is removed, it's rock solid when the foregrip is on.
These are single shot, break action rifles and I don't think it's dangerous to shoot a barrel that wasn't factory fit. Your accuracy might not be good, you might get light primer strikes, you might get no primer strikes if the chamber isn't in alignment with he firing pin, but if I had a Handi Rifle barrel that I knew wasn't factory fitted to the frame, it wouldn't stop me from trying if I could get a spare barrel cheap enough.
The issue is that there are no guarantees like with Thompson Center. I decided after I got mine that it was going to be the only Handi Rifle I was going to buy. Since you already have yours, I would be happy with what you got and if you want a single shot platform with interchangeable barrels, go T/C or wait for Midland Arms to get their crap together for the Backpacker.
I wouldn't hold my breath on Midland tho, they said at SHOT 2018 that rifle barrels were coming by the end of that year.
It's almost 2020 and still nothing.