SA is a physical attack; it won't even stack with elemental WS like burning blade or wasp sting. So, unfortunately, you can't stack SA with any Ranged WS.
I'm a hume, started as thief and am now working on my RNG. Some things to keep in mind.
Rangers are expensive; thieves are rich. The two basically work out to breaking even, except when you consider the next point.
THF(25+) and RNG together make *the* Notorious Monster hunter. You've got Wide Scan, Flee, Treasure Hunter, and all that glorious DMG you can do. Take advantage of this combo.
THF(15+) has one of the highest single melee hits in the game. THF(30+) with a TA/SA/WS is probably *the* highest melee damage in the game. RNG has the highest dps in the game, even higher than BLM. RNG also doesn't get any WS between 30-ish and late 50's. So, make 8 ranged shots and then fire off a SA/WS with your dagger, THEN hit them with an Eagle Eye Shot. Any mob that can survive this you probably shouldn't be fighting.
People love thieves after 15 for TH, even if they don't know the benefits of SA and TA. People love rangers at most levels because they are *the* puller and have such high dps (if properly equipped). Put the two together and you won't have to wait long for parties.
Rangers will get aggro; thieves' evasion bonuses will help to keep you alive. Thieves don't do much damage in normal combat; ranger WS will help to keep you alive.
The Job Traits and Abilities even pair up well - Steal and Treasure Hunter with Scavenge; Flee with Wide Scan; Hide with Camouflage; Evasion Bonus with Caution.
I could go on (and on and on and on...). Obviously, I like the pairing
There are probably disadvantages, or more realistically advantages that other combinations have over the RNG/THF THF/RNG combos. To each his own.
Whether you choose to go with RNG in the long run, stick with THF to at least lvl 30; TH and GF really help with obtaining all the items you need for quests, missions, etc. WHM may be the saviors of partying, but THF are the go-to guys when people need quest items.
Good luck and good hunting.