I found the helm, boots, gauntlets (all light armor) and a heavy armor shield. Can I find a light armor cuirass in the castle somewhere?

No, but you can buy them from the blacksmith later on. If you are playing on the vamp side, then you have to loot them from random attackers.

Absolutely if you are a vampire you will end up with enough armor and basic weaponry to open your own Dawnguard supply depot. And strangely enough the one shield my vampire character has duplicated itself with the help of the magical mannequins, yet would not for my Dawnguard character who had to go buy her extra shields from the Dawnguard smith. Bit miffed by that.

I could dress a whole vampire army with all the robes I have from the slain. lol.

My vamp character collects all dawnguard armor and weapons as trophies.

Braggen would need at least another 6 new houses if he was going to collect them all as trophies, he just leaves them there now, if the weapons stay and clutter the place up he collects them sells them. Just about every NPC in Skyrim has a Dawnguard weapon from Braggen, he had plenty to spare. Pity you could not get the NPC's to wear the armour, they could have ALL been Dawnguard.