No, you need to keep the drives consistent in the tier in a Pool. IE, same size and speed. Not sure why you would get 2 x 300GB drives, unless you have a particular failure/isolation domain requirement.  Without knowing all the details, I would create; 1 Pool with 20 x 900Gb SAS 10k drives in a RAID 5 : 4 x (4+1) 1 Pool/RG with 2 x 300GB SAS.  Not that many other sensible options with those disks.
Hello! I have new Storage VNX 5100 with 2 SAS HDD 200 Gb 10k and 25 SAS HDD 900 Gb 10k. I worked with very closer storage and I know how used it, but I can't find best way how create storage pool with this set HDD. Please help me, how I must create pool with very good characteristics for capacity and performance. 
Pool for VNX 5100