Your config would be fine. The PERC doesn't really care about where the physical drives are on the backplane for what it does. Keeping things together is really for the human benefit. If you're concerned, the easiest thing to do would be to create a singular RAID volume for your OS, install to it, then deploy the rest of your VDs after. That would eliminate any confusion when you go to deploy your OS. 

You might also consider a hotspare drive, considering the amount of drives you'd be populating the system with.

You could have an external controller, but the backplane won't have another connector set for another internal controller. You should be able to confirm that by removing the cover and checking the connectors on your backplane. However, I really don't think you'll have any issues with the one controller, unless you your config uses a very high amount of disk IO. Using SSD storage would probably be a better option than having an external storage enclosure paired with an external PERC.