We advise to keep LUNs in the same aggr roughly the same size. What can happen over time if you have, for instance, 10x 100g LUNs and one 400g LUN in a single aggr, is that as the 100g LUNs fill up, there will still be 300g of free space, but it will all be in that single LUN. The result will be what we kindly refer to as "uneven" performance. Once those smaller LUNs fill up, they are no longer handling write IOPS. It's certainly possible, and there are people doing it. But it can (and probably will) cause performance problems.
You are correct that most of the planning guides presume dedicated array RAID Groups. Our best practice is to not share RG's or ports with other array hosts. Again, the concern is "uneven" performance.
You should figure out, for each array RG, how much space is available to us. Based on that, we can figure out the best way to carve it up. Feel free to follow up with me over email with some more details, and I could offer you some more specific advice.
Technical Marketing Engineer - VBU