Mixing 3.5 hdd and 2.5 hdd in LSI RAID 1

If using the same connection protocol (read: SATA), it should work. However, 5.4k RPM disk are consumer (laptop?) ones, absolutely not adequate for any production RAID.

In other words: if the server does any real work, don't do that. If for lab/testing, you can try.

Usually, the array runs at the speed of the slowest disk, so putting a 5k 2.5" in a 7k 3.5 array probably more than halves the speed.

Also, some RAID controllers particularly don't like mixing disks of different speeds - performance may be impacted even further. You'll have to try though, no vendor explicitly supports this kind of setup.

As shodanshok has already pointed out, for production storage you'll want to match disks.