Creating expressions to filter the source combinations in PCM designer

Hello Experts,

We have a requirement where the client wants the project-level allocations (currently performed in PPM) to be migrated to PCM.
However, the list of projects and their associated tasks comprises approximately 400,000 members (around 250,000–300,000 projects, with each project having multiple tasks roughly another 100,000 members). This volume is expected to grow by an additional 20-25K projects and tasks each month.

a) Is this a typical or conventional requirement where PPM-level allocations are handled in PCM? If yes, how can we manage potential performance issues?
b) For most of the allocation rules, the existing source filter conditions include patterns such as picking accounts ranging from 10* to 40*. Is there an efficient way to handle such conditions while configuring standard or custom allocation rules, rather than to create an alternate hierarchy that groups these accounts - as it has an additional maintenance effort involved.