Well i am also a galka mage not just smn but i play smn, whm, and blm.
And the biggest thing is use good/great gear that being said you do not need astral rings to survive post lvl 10 just lvling up cooking or buy juices and roast mushrooms/whitch kabobs, not only will you save yourself a big upfront cost.
you will also get a better rep becuase you are prepared for battle which is more than i can say for a lot of whm i have PTed with.
Also at higher lvls against gobs do not be afraid to use Divine Seal + Curaga if it is needed, simply put you can take the hits that no other whm could ever take.
As for gear I advise you to buy Zelot's Mitts at 11. I say buy because you will have trouble getting the NM since he also drops the monster signa and the ******* is always camped.
then camp Ashmaker gotblot for a Priest's Robe he is under camped the most i ever saw was 4 people other than me there. after those 2 you are pretty much free to get any good MP boosting gear that you can afford and you will be set until you start getting your RSE which once you have your first RSE (gloves) you will have more mp than a hume without astrals.
As to Galka Mage Haters well look at them like i do they are just jealous since you are actually succeeding at something that is very hard to do.
Also once you hit 21 Regen will be your best friend spell wise. and don't play dumb taru whm mode in other words don't waste MP casting Cure II when spamming Cure I is more than enough.
If you do go brd/whm i personally thing you should try to get at least 1 astral ring becuase galka brd/whm mp sucks and personally i feel bad when i can't do much to help out the healer.
Now for the most important thing make sure you know everything about your job before you go into it. and be prepared for every lvl. keep your spells current well at least spells your party needs. if walk into a party as a smart galka whm then you will get respect when its deserved and also usually convert stupid racists.
If you are unsure about something in the party, ask you lose nothing from asking and if you get stupid remarks well you might as well leave cuz that is the first sign of a bad/slow PT.
Also when you get to 30 i would suggest unlocking SMN and lvling it for your sub. it will give you a great MP boost. at 25 it is the same as having 2x Astral Rings, in other words SMN at 25 will give you 50MP+ while BLM will only be giving 35MP+
lets see what can i say, oh ya have fun. and don't beafraid of stealing aggro to save the PT it will impress a few, scare a lot but saving the PT is the most important so do whatever you can in emergencies.