There is a woman in the Black Market before the 3rd gym who will evolve trade pokemon.
Scyther, as you said, is a good pick, as it evolves into Scizor right before the third gym, and is very helpful in the fight immediately after you get it. It also does very well versus the Elite 4. Rotom is an okay pick which is great in the early game, and can be useful once you unlock the forms later, wouldn’t be my first pick for a ghost type, that being Gengar. You can get a Gastly in the initial forest, and it evolves at the 3rd gym, and stays relevant and useful for the entire game, well, at least most of it. I highly recommend getting a Magikarp with the Old Rod, which is helpful for pretty much the entire game. Some other good water types are Feraligatr(you can find Totodile on Route 2) and Delta Haxorus(D. Axew found in Miara Town).
I recommend getting a Zubat for Crobat, a great underrated mon with a good typing. Both it and Abra, another great pick, can be found in Dark Cave off of Route 1. Drillbur, when combined with Tyranitar, or using its HA is a good find in either Midna Mine, Utira Town’s trade, or the Friend Safari. Durant can be found in Midna Garden, but it’s outclassed by Scizor. Magnemite is a good Electric/Steel type found in Cyan Cavern, helps quite a bit versus a later boss. Axew is a good pokemon, found in Broadcast Tower(Helios City)'s Mission 1. Ralts and Kirlia can be found in Route 3, Choice Scarf Garde is very useful(Delta too!), as PeterHolmes74 would testify. Cyndaquil can be found on Route 4, after breeding, it can be a very useful Mega. Sneasel can be traded for in Vipik, but requires breeding to be useful. You can find Fennekin in Crossroad’s Hidden Grotto, another underrated pokemon.
In the Rezzai Desert, you can find Krokorok and Vibrava, however, I wouldn’t use Flygon unless Mega, as the Armored and Normal forms are both underwhelming. You can also find more Axew here in the Rezzai Cavern. Volcarona becomes available in Roggan Town, a ghost type can be traded for Larvesta here. Item Farmer Pumpkaboo can be found on Route 7. Utira Library hosts Gengar and Litwick, powerful ghost types. As Boggaspotatoe said, D. Roserade is another good pick here. Kangaskhan, Chansey, and Houndoom are found on Route 8, as well as the beast that is D. Snorlax(actually D. Munchlax) in the Hidden Grotto.
Friend Safari mons
Some good slot 1 mons are Taillow, Starly, Slowpoke, Pumpkaboo(good for Heart Scale farming only), Magnemite, Helioptile, Mr. Mime, Espurr, Seadra, Shedinja, Inkay(Item/Cash farming only), and Clefairy.
Hmm, guide? Nah, any viable mon works IG.