The iron rust is unlikely to be a problem to the plants. The rusting will accelerate with plants so you may find the containers falling apart after a short time if the metal is thin or already mostly oxidized.
If you paint, be sure to ask for a non-toxic paint.
I have an old coal bucket that I plant begonia's in and the poor thing is falling apart from rust, so I've been doing what Lilith suggested and it works fine, and I still have the effect of the old pot.
you could always line it with polythene too, that would work, just as long as you put some drainage holes in. i think the coal bucket would look quite attractive falling apart like that, just make sure you get a pot close to the colour of the container as you may see it through the holes, lol