OK well, thats not really addressing the point of the thread....
The point of the thread is not whether you can trade different markets, which obviously you can of course, but rather if trading very different markets can negatively impact on your performance on one or more of those markets...
Obviously it takes time to learn any market, and you should stick with one until proficient. However, there may be merit in trying an alternative market if its not working out on a particular market...provided you have given it adequate time, and have taken a systematic approach...
I would consider trading stocks and in particular trading momentum plays, as one market....
In terms of forex I'm not sure that newbies should only stick to one market ie one currency pair....although there may be benefits to doing so :
If a particular set up is being traded such as a trend following approach based on daily / weekly timeframes with entry and stop positioning based on price action on lower times frames, then why limit your search to just one pair if you can find good opportunities in a wide number of pairs.....