Yes, best practice is to make all packs in the series built with the same "capacity" as possible. All cells/packs in series should fill up at the same rate. So if the lowest capacity pack is filled first, you have wasted capacity that will never be used, and/or you will over charge your lowest ones.
Llike Korishan & Wolf says, packs in series should have the same/similar capacity.
You should mix up your cells across your packs so each pack has some higher + some lesser mAhrs.
Use repacker (see link top right)
Yes, P-clusters put in series must be the same overall capacity per cluster.
Like let's say we need to end up with a 3-series.
1st pair can be a 3000mAh cell and a 500mAh cell in parallel - total 3500
2nd pair can be a 2000mAh cell and a 1500mAh cell in parallel - total 3500
3rd pair can be a 2500mAh cell and a 1000mAh cell in parallel - total 3500.