Nico Williams is younger than Lookman and he has two big advantages against Lookman: his experience in La Liga is what Lookman does not have and will need time for adaptation to a new league; and his understanding as well as good combination with Lamine Yamal in Spanish national team recent years. It is another disadvantage of Lookkman who will have to get familiarized with Yamal and build up bonds with other new teammates.
I believe transfer prices of Nico Williams and Lookman are not too different and both are expensive. However perhaps Nico Williams will cost Barcelona more money for his wage.
It will be interesting to see how he adapts to a new team.
Unfortunately, it is not a given that great champions play well together, there are many reasons and they are all strictly human reasons
for example, Messi had said that during his time with Barcelona he got along very well with his teammates, in fact he was unstoppable