I want a program to make it so /home and Mydocs will grow or shrink according to the usage, sharing the same free space.

One way i picture it working would be to have both partitions actually be files in a third partition, those files would grow and shrink, and the program would insert "false" information about the free space on each partition representing how much space is left in the third partition; this program would probably need to kick in quite early during boot process since /home has things that are necessary for booting (or so i've been told) . If the file systems in question don't work well with being resized frequently, instead have the those virtual partitions believe they use all the memory (twenty somthing gigs), and have a file inside them that has a few MBs less than the currently avaiable free space, the file wouldn't actually be occupying any space but the partitions would react is if it was, measuring free space would be done by checking the size of the file and adding tot he reported free space so it wouldn't show true free space in most programs, having a bit less than the actual free space will help with fast writes providing a buffer of free space that is always avaible so programs won't think there is no more free space untll really there is no more free space; the size of the buffer should be configurable so if a bigger buffer is necessary (like for downloading disk images with a few gigs) it will show it has enough space for that.

Regardless of how ti's done, it is important that it works when booting, works when plugging into USB, and doesn't loose data much less brick the device when flashing (and if possible is easy to reactivate after reflashing).