Re the compatability with the Leatt. I've seen riders race with pretty much all types of armour, usually over the top of their jersey, whilst also wearing a Leatt. However, they sometimes have to cut little bits of their race shirt around the neck area if it's the full on plastic type like say the Thor Aftershock, but I think you'd be pretty much ok with any type.
I suppose some would be a lot more comfortable than others, so if poss you might be best trying the armour and Leatt Brace on together, so you know what feels most comfortable for you. If you then go back to buy the Leatt from the same shop once you've saved up enough money they might knock you a little bit off too.
Good luck
