The only thing I've heard that is restricted is playing multiple accounts on one device (some people get around this by changing their phone ID). Those that try trading that way see their familiars disappear, from what I've heard.
On the other hand, people that play different accounts for each device (for example, two accounts, one each on iPad and an Android at the same time) seem to be fine, however.
I want to start a second account using BlueStacks but before I do, I want to know what's ok and what's not ok (stuff that can get you banned).