Are there any restrictions on what a trading strategy can do?

It is essentially up to you, what you would like to consider as one strategy

A strategy can trade multiple instruments, multiple asset classes, multiple time frames, multiple brokers and multiple accounts

Regarding the trading logic there is no limitation. A strategy can be something very simple or something very complex

However please note that the entire performance and reporting functionality of AlgoTrader happens on the strategy level. So if you would like to see performance charts and performance metrics on an instrument level you would have to instantiate multiple strategies

Also, if you need to start/stop individual functions separately, it is best to put them into separate strategies

On the technical side each separate strategy allocates a certain amount of overhead (memory and CPU). For that reason it is best to combine functionality into as few strategies as possible if there are no good reasons not to separate them