1) There is no limit to the number of packs you can charge in parallel provided you use the same cell count. You can hook as many parallel boards into a daisy chain to add more packs in the circuit if you wish.
2) Be sure the voltage of all the packs is relatively close when hooking multiple packs in parallel. Ideally you want them close to storage mode to make it simple. If you hook up a fully charged pack up with a fully discharged pack, then voltage will automatically self balance between the 2 packs so they reach the same identical voltage which can create a burst of current which may damage the board as the fully charged pack pushes the higher voltage into the empty pack.
3) You can mix/match capacities and C ratings between various packs, just don't mix the voltage (cell counts)
4) Simply add up the total capacity of all the packs to figure out the recommended charge rate. For example lets say you have a 5000mAh pack + a 6000mAh pack then that works out to 11000mAh or 11Ah which would be 11A at 1C or one times the capacity of all the packs. If the packs are rated to be charged at 3C then that works out to 33A of current you can charge them at.
5) I prefer to set my LVC on the ESC to 3.5V/cell under load which works out to 3.8V/cell at rest which is perfect for storage mode. Simply run each pack down to LVC to make it simple to combine packs together for parallel charging
Personally I keep away from parallel charging.
I own and use 3 chargers, where one is dual charger.
A SkyRC, a Maxxtrax and a HTRC (Dual, which is a very nice device).
In FPV drone land I've seen some destruction and fires happen which made me a bit lipo paranoid.
Be careful with this and pay close attention when hooking up your chain.
Use a GOOD quality para-board, spend some extra bucks there.
Do not just use a lipo safe bag, use something safer like a vented steel box.
IMHO, you should NOT mix different capacities, but that can be my paranoia.
And one would need a very powerful charger to achieve 33A charge rates.
Just my 2 cts ;-) Be safe and have fun.