Tom-- I'm glad you enjoyed my document. 🙂
To answer your questions--
The decoder is very flexible about what meta keys parsers are allowed to create. A parser is allowed to populate any meta key it likes, even if it's not in the decoder's custom index definition. The decoder keeps track of the meta key names generated by the parsers while capture is running, and uses that to provide the list of meta keys. As you have found, the auto-complete functionality is probably the only area where you might find some benefit from explicitly defining meta keys in the decoder's index configuration. As you can see, if a parser hasn't generated the meta key, it won't be able to provide the meta key to the auto-complete functionality.
Regarding the number of index slices-- this is a good question. The answer is that it depends on how much retention your index has. For example, on a concentrator the index has a fixed size and automatically rolls over after a period of time. A lightly loaded index will take a long time to roll over (maybe months or even years) while a heavily loaded index will rollover in a matter of weeks or days. If your index is heavily loaded and rolls out quickly, then you will see that the number of slices in the index always stays low; perhaps 100 or 200. On a lightly loaded index the number of slices may be in the thousands. For Concentrator 10.3, my general rule of thumb would be to try to keep the number of index slices below 500.
