Jirachi was released in the same game as the first Psychic/Steel so you could say that it tied as the first of that type combination. Electric/Ground would also be a nice type for it.
I wouldn't be surprised if it was Electric/something... Gamefreak really loves electric rodents. I guess they figure, "Worked once, works every time".
There are a few things that Serebii didn't mention:
Rankurusu gets Psywave, not Psybeam
Rankurusu gets Dizzy Punch
Gochiruzeru gets Faint Attack
Chiiramii gets Swift and Wake Up Slap
Piggie bacon gets Take Down
Also, Sweep Slap seems to do a lot of damage, and it has 10 PP as well. The chances of Sweep Slap becoming a Normal type Rock Blast is getting more likely...
Gochiruzeru's HP appears to be somewhat low, taking about 50% from Psywave.
So, some conclusions we can draw from this:
Rankurusu sounds like it will have a large support movepool with Psywave and Hypnosis confirmed. And I'm very surprised it has Dizzy Punch, this means although it's attack stat is poor, it'll be able to learn all the Punch TM and Tutor moves.
Gochiruzeru learning Faint Attack is very
very interesting... could it become Psychic/Dark with a future evo!?
Chirami's Sweep Slap does seem like it will be Rock Blast/Bone Rush, I agree.
Pokabu learning Take Down hints that it will be physically-based and likely have a strong Attack stat.