I recently picked up a finecast Carnosaur to replace the old metal one, which was rather poorly assembled and painted due to me being new to the hobby at that point. I noticed something very interesting - the finecast guy, despite being the exact same model, comes with a long monster base instead of the 50x50 mm base the old guy came with. Having the smaller base could be a big deal with it comes to flank charges and being shot head-on by cannons and the like; can I use the square base since the model used to come with it, or am I forced to use the one included? Would it make a difference if I kept using the metal guy? I'm not going to re-base him to make him 'legal'.
You should be fine. That thing's tiny anyway, there's no reason for it to be on the bigger base. I'm using the 50x50 on my finecast one too, seeing as my identical metal ones are on them.
BRB Page 80 - says it should "always" be mounted on the base it is supplied with. That said, the model used to be supplied with a 50mm base, so no one will care. And if they do, do you really want to play against them in a friendly? Tournament wise, just say it was supplied on a 50mm base, maybe?
Heck, say that you thought they messed up and gave you the chariot base by accident. I mean, otherwise you could play it on a 20x20 if they accidentally gave you a bunch of those instead. GW really needs to include base sizes in statblocks.
The new carno is indeed on a chariot base, you can mount it on the old base (beware RAW monkeys) but it works ok for me on the current one.