Do I need a special crimper for these terminals?

You'll want a quality spade terminal crimper — most auto-focused crimpers at any parts store handle 1/4" terminals. Hand-crimping is possible but won't give you a reliable, repeatable connection. If you're doing a lot of wiring, invest in a real crimper.

Can I use these with solid-core wire or only stranded?

Stranded wire works best with crimp terminals — it grips better and won't break when the connector flexes. Solid core can work, but you'll get a more secure hold with stranded of the same gauge.

What gauge wire fits into a 1/4" spade terminal?

1/4" spades typically fit 14–16 gauge stranded wire. Check your specific crimper's chart to confirm — each tool has a range, and matching wire gauge to terminal size is what makes the connection solid.