Wei didn't need Yellow river basin to gain some man power. They was very strong and since they could beat Rouran's, their cavalry is very formidable, I say most strong at the moment.
I didn't find any historical source that Wei got military assistance from Later Qin, which that time was almost at chaos.
So why was the Wei able to only gather 20,000-30,000 troops when attacking the Yan during the battle, who had up to 100,000? The Wei was heavily outnumbered when both attacking and defending, which was why they were forced to continually retreat earlier in 395 when the Yan initially invaded, instead of confronting them directly. In terms of the battle itself, here's a passage from the Zizhi Tongjian, which Wikipedia relied heavily upon, and provides the troop numbers and specific events:
燕軍至參合陂,有大風,黑氣如堤,自軍後來,臨覆軍上。沙門支曇猛言於寶曰:「風氣暴迅,魏兵將至之候,宜遣兵御之。」寶以去魏軍已遠,笑而不應。曇猛固 請不已,麟怒曰:「以殿下神武,師徒之盛,足以橫行沙漠,索虜何敢遠來!而曇猛妄言驚眾,當斬以徇!」曇猛泣曰:「苻氏以百萬之師,敗於淮南,正由恃眾輕 敵,不信天道故也!」司徒德勸寶從曇猛言,寶乃遣麟帥騎三萬居軍後以備非常。麟以曇猛赤妄,縱騎遊獵,不肯設備。寶遣騎還詗魏兵,騎和十餘里,即解鞍寢。
魏軍晨夜兼行,乙酉,暮,至參合陂西。燕軍在陂東,營於蟠羊山南水上。魏王珪夜部分諸將,掩覆燕軍,士卒銜枚束馬口潛進。丙戌,日出,魏軍登山,下臨燕 營。燕軍將東引,顧見之,士卒大驚擾亂。珪縱兵擊之,燕兵走赴水,人馬相騰,躡壓溺死者以萬數。略陽公遵以兵邀其前,燕兵四五萬人,一時放仗斂手就禽,其 遺迸去者不過數千人,太子寶等皆單騎僅免。殺燕右僕陳留悼王紹,生禽魯陽王倭奴、桂林王道成、濟陰公尹國等文武將吏數千人,兵甲糧貨以巨萬計。道成,垂之 弟子也。
And here's a passage indicating that the Wei relied on its neighbors even
after the battle in order to maintain an offensive position:
五月,辛亥,以范陽王德為都督冀、兗、青、徐、荊、豫六州諸軍事、車騎大將軍、冀州牧,鎮鄴;遼西王農為都督並、雍、益、梁、秦、涼六州諸軍事、并州牧, 鎮晉陽。又以安定王庫辱官偉為太師,夫餘王為太傅。甲寅,以趙王麟領尚書左僕射,高陽王隆領右僕射,長樂公盛為司隸校尉,宜都王鳳為冀州刺史。
In other words, the Wei was nowhere near a position to assume a dominant stance until it first scored a decisive victory over the Yan, then take over the rest of North China after vanquishing the Later Qin. These events occurred in 395 and 417 IOTL, respectively, so a different outcome in the first battle ITTL would butterfly these results away.
At Canhe Slop Battle, Wei was attacker and Yan was defender. Which means Wei was rising and Yan was surviving.
This essentially means nothing in context of the battles as a whole. The Yan was on the offensive for most of that year, and Murong Bao only retreated on November 23 due to adverse weather, not because he was suffering significant losses against the Wei. In addition, Tuoba Gui was forced to entice the Yan into retreating by capturing its messengers and forcing to lie that Murong Chui had passed away, as he was hoping to hold out until the invaders were forced to retreat. The battle at Canhe Slope was a large gamble by Tuoba Gui in the hopes that Murong Bao would be caught unprepared, which succeeded IOTL, and he was forced to slaughter most of the Yan forces who had surrendered, which outnumbered his total forces, suggesting that he had nowhere near the capability to equip and support troops that were originally not under his control.
To sum up, the Wei's decision to attack the Yan was essentially a huge risk that could have easily gone the other way, instead of part of several battles and/or campaigns which the latter had continuously lost, causing a gradual power shift over time. In fact, Murong Chui conducted another campaign in 396 into Wei territory, and won several battles, along with killing Tuoba Qian. In other words, if the Wei had suffered a loss on Canhe Slope due to Murong Bao's preparation beforehand, then the Yan would retain control over its possessions, and the Wei would no longer be in a position to regain the offensive until Goguryeo would eventually interfere.