What problem are you trying to correct?

I ran and did aerobics in green Superfeet for a couple of years, wound up getting custom orthotics made, which are about the same firmness. There's definitely an adjustment period (they told me to phase in new orthotics by alternating with my old ones for a week or two), but I've never had to physically break in a pair.

You can layer a thin cushion insole over an orthotic as long as your shoes are deep enough for both. I suppose if you wear your shoes loose it might slip, but mine never did.