Linux – Software RAID 5 with different sized hard drives

There shouldn't really be any isues mixing disk vendors, Linux software raid is discussed pretty thoroughly here, you shouldn't have to do anything differently just because the drives are from different vendors.

"There are no other special requirements to the devices from which you build your RAID devices - this gives you a lot of freedom in designing your RAID solution. For example, you can build a RAID from a mix of IDE and SCSI devices, and you can even build a RAID from other RAID devices"

(From the Linux raid wiki)

As far as setting up the raid during the installer goes, you have to use the "alternate" (textmode) installer, full instructions for this are here on the Ubuntu wiki.

I would always try to stay as similar as possible, so I suggest you to buy from the same manufacturer and hard drive family if possible. Just buy a bigger one.

If not possible, then sure another manufacturer would do it, as long as the disc is exact the same size or bigger, never smaller! Be careful, just being one sector smaller than your old discs will make it not work if your RAID uses the full old disc size. That's why it's recommended while creating a RAID array not to use the whole disc size, but leave some percentage unused, then it's simpler to use different disc with "same" size which have not exact equal sector count.

I have myself some mixed RAID6, Seagate and Hitachi, no problems. But it's not a LSI controller, it's an Areca one.