Here is a sample capx which pick a random item from a table ( rex_firebase_itembook plugin )
The concept of random picking is
1. shuffle items by set a random string at a key ( "filter" in this capx )
2. get 1 item from the first or the last item of table which sorted by that key ( "filter" in this capx )
3. reset random string to put this item back
BTW, I am thinking about making new plugin to separate the features of rex_firebase, because that rex_firebase is too large...