Stopping the flame tank rush is just as simple as using GDI's power to dominate the battlefield. The only time a flame rush seems to truly work is when there is no tanks in the field...if there is, you'll find that the flame tanks will either retreat to their engies after being damaged, or attack you and either get destroyed or heavily damaged before your tank is destroyed. Just get two MRLS's in the field relatively near each other and close to the base entrance so that one Hotwire can repair both. Then have two medium tanks to help guard the MRLS's and clean the battlefield. Whenever they need repairs, they can just go to the MRLS's to be repaired.
All of these uits can be bought with 800 credits or less, which is the cost of the flame tank. If you continually dominate the field, NOD will be too busy trying to clear you from the battlefield to flame rush. And if they do try to rush, you can damage them heavily, then return to base when and if you are killed.
Honestly, all it takes to defend a teamwork flame rush is to beat it with teamwork defense. If you can just convince the people in the server (including yourself
) to not try to only buy APC's for rushing and Mammy's for power, then you should be fine.