3/32' v 1/8' ?

You can run a 3/32" cog or chainring on a 1/8" chain, so it would be fine to have a 1/8" chainring and chain with a 3/32" freewheel.

So if I buy this shimano

http://www.wiggle.co.uk/p/cycle/7/Shimano_BMX_Freewheel/5300005982/

with 17 or 18T I won't have a problem with my current crankset? althought the chain has wider spacings?

ok, it's set then :)

thanks for your help.