I have an FOX set that I love for as3 (4/6 wings), but can i use the same setup for fe3?

If I keep the ratio of downlad the same front/rear how much can I go before it starts to have bad effects in other ways?

The more downforce you add, the more grip you should get, but too much downforce will start to reduce the speed on straights, so you should find the correct ratio that will have grip and won't reduce the straight line speed.

Nice avatar btw

Cheers Imulda, but how to know when then the suspension is bottoming out/?

You can do some laps, save a replay and then watch the forces (press F). If the orange bar goes red, there is no travel left (at least I think it works like that).

I wrote a replay some time ago about wings vs. speed that might be of interest here: http://www.lfsforum.net/showthread.php?p=1620313#post1620313

You can also download the replay analyser and see the remaining suspension travel as a continous graph to study it in more detail than what the forces view ingame allows. You can download one of the analysers here: http://www.lfsforum.net/showthread.php?p=1368765#post1368765