Where instruments come from
The catalog is filled from the collector's data: an instrument with no candles does not appear in it. That protects against the obvious trap — picking a pair, starting a run and getting an empty result.
History depth
Each instrument has its own, and it is shown when you pick one. A one-year run on an instrument with twenty-nine days of data is impossible — better to learn that before starting, not after.
Depth depends on when the instrument was added to collection and on the timeframe: there are sixty times more minute candles for the same period, and they are kept for a shorter time.
How many instruments to take
Start with one or two. Every added instrument is a separate stream of signals, and the decision on each is yours. Eight instruments on the hourly timeframe produce a stream that is hard to consider carefully.
Disabled instruments
An administrator can disable an instrument — for instance if data collection for it stopped. Renaming in the catalog is deliberately absent: the symbol identifies the instrument in configurations, in candles and in signals already issued, and renaming would make history meaningless in retrospect.