There are all sorts of irregularities. I am sure your vet has your dog's best interest at heart.
You would have been devastated if he didn't tolerate the anaesthetic and then found out that the vet had heard something but decided to not investigate it first, surely?
The fact that castration is common has no bearing in this at all. It is the anaesthetic which is the risk. Any general anaesthetic, whether in dog, cat or human, carries an element of risk, even if it is to remove a grass seed from an ear.
My advice would be to stop searching such a vague term as "irregular heartbeat" on the internet, go for the specialist appointment and wait for his advice.
I know some people are only out to make money, but I don't think it is safe, nor desirable to suspect everyone of this. I may be niaive, but I still consider that vets enter the profession because they care about animals. If their sole aim was to make money, I can think of hundreds of ways which don't involve one of the longest degree courses at uni, long hours and all the other undesireable parts of veterinary medicine.
I hope that Sammy gets the all clear from the specialist and can go ahead and have his op, and a long, fit future.