Is it safe to make Jello in disposable aluminum pans?

You could, of course, use aluminium (British spelling) foil to line a pan. But the foil may become rather creased and crinkly, even getting a hole in the foil. Into the creases, the jello (Jelly) will flow into these creases and through any holes. Having done such a thing in the past - I recommend using the pan, without the foil, to melt the jelly. Or, better still, pour the liquid jelly directly into any moulds (British spelling) or small plastic containers you wish to use.