yes, the top hat cams for the T&S will work in the 503. The exception is the basting cam, as it has the ability to skip stitches, and the 500 series will not do that.
I think the flat cams are the same between machines too. the 400 takes top cap cams, not the flat ones that the 300 group (like 309, 319, 328 take). the 400 takes the same as the 500 and newer groups.