Who to farm next f2p

what fleet lineup am I shooting for before malevolence

You want all 3 Geos on the field together. If Hound's Tooth is strong enough, then you might want HT, Sun Fac, Soldier as your starting lineup & Spy as your first reinforcement. Other reinforcements would likely shift depending on what you have available, but you also want to farm Hyena when possible (and earlier is better), and the Vulture droid is helpful once you actually have Mal. You don't need to use vulture before you have Mal, but you will need to start farming it a couple months before you get Mal so that it hits seven stars about the same time as Mal hits five. As long as it's unlocked, though, that's the important thing. Because of RNG it's impossible to estimate these things exactly, so as long as you hit 7-star vulture within about 30 days of unlocking Malevolence (it unlocks at 5 stars), then you timed it well.

I am also undefeated in grand arena I was wondering if this plan will boost my GP significantly

Obviously it's better to win than lose in GAC, but the rewards there are small compared to the rewards elsewhere. Your GP will have to go up as you progress in the game, so my advice is not to worry about it. As long as you are focussed on worthwhile toons, things will work out. Eventually you'll lose some GAC rounds, but with a good development plan you'll always win more than you lose. This one basically takes care of itself.