You ask some pertinent questions.
Why would the saleman tell you to use .451s?
Well, .451's will work but not as good (IMO) as .454s. The saleman may have had a bunch of .451 bullets he wanted to sell but no .454s. The salesman may be ignorant of cap and ball shooting.
As regards the mold, depending upon which mold it is, you may get bullets that are actually larger than .451. I use mostly Lee molds and get bullets up to three thousand's larger than the mold size.
Another technique is to drop the bullets out of the mold into cold water. This will help them retain their larger size, rather than to shrink as the cool slowly.
Cast some bullets this way and see if you shave a ring all the way round. If you do, you are prolly okay.
If you don't shave a complete ring, then you are relying upon the wad under the ball (if you use one) or the lube over the ball (If that is your technique) to provide the seal on the chamber in addition to the lubricating functions they are intended to produce.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson