I don't see why you could play on Pokemon Online and not on Showdown, but as long as your opponent agree, you can play on Pokemon Online. Just be sure that you still follow Smogon tiers. Also you can't force him to agree.Contacted my opponent
and well I like to post a question, I can not play pokemon showdown because I'll be traveling and long even think I will be unable to connect for two days, I can play for online pokemon, if I may?