Ambulia IS a weed haha. I have half my 50g dedicated to my 'ambulia forest' and I love it! It grows soooo quickly, and I have often cut some off and just chucked it in a spare tank where it grows beautifully as a floater (but does get roots shooting off everywhere because it's not oriented). I think it'll do great, but you may need to keep it trimmed for your crypt's sake. Having said that, I don't think crypts need too much light anyway, so it may be fine. Wing it
If for some reason it fails or you get sick of roots, you could probably plant the stuff and let it grow so long it bends over in the water and acts as a floating plant anyway
Mine does that a lot haha.