Can I play this with just Sparkstalker on my team and no other Firecons?

You can play this with just Sparkstalker or with one other Firecon or with two other Firecons. However many you have: that’s how many the second ability will apply to. The Stratagem is Common and uses Sparkstalker as its named character so that it is legal in the Junkion format, even if the rest of the Firecons aren’t.

Does the first line of text apply to my starting team or just my battle deck?

This does not apply to your starting team. “Your deck can…” is an ability only covering cards registered in your battle deck.