Can I play a card with no shield icons as a shield card and have it gain 1 shield form the Fireblade's ability?

If so, can I play a card with no shield icons as a shield card even if he isn't in play?

No, his ability is in Interrupt, and therefore requires a triggering condition to use. In this case, the triggering condition is using a Tau card as a shield, which requires it to have printed shield icons.