I would seriously advise against smoke units in HO. First, the "smoke" is atually a vaporized oil, and that oil will redeposit itself all over your layout and track. You will be cleaning your track more often. Second, the smoke never looks very realistic, and in my opinion looks so fake that it actually deters from the realism. Third, good smoke units get hot enough that they may actually melt the plastic boilers of your steamers if they are ever ran without the smoke oil. If you were to install a smoke unit, it wold be a good idea to make somme kind of on/off switch to turn it off when you don't want the smoke.
kevin