For mulliganing, it does depend a bit if it's game 1 or 2 and whether or not you're on the play or draw, but for now let's assume it's game 1 and you're on the play. The ideal hand would be a land, an accelerator (like spring leaf or moxie opal), a couple creature and a Haymaker (any non
vault skirge 2 or 3 drop).
Inkmoth nexus is also a very good reason to keep a hand since a lot of decks struggle to deal with it and it can be such a fast clock.
Really what it comes down to with affinity is:
With a keepable affinity hand you should be able to look at it and know how it's going to win the gameAffinity is such a fast deck that what will kill your opponent is most likely in your opening hand, and keeping a hand without a clear way to win is risky since most other decks will be able to get ahead if you in the late game. The good news is you can mulligan pretty freely since your deck is already so explosive and able to play so much so quickly.
So a keepable could be like:
1
inkmoth Nexus1
ornithopter1
memnite1
mox opal1
cranial plating1
darksteel citadel1
galvanic blastBut if that
cranial plating is a
signal pest... I'd now probably mulligan, because the hand doesn't have a clear path to victory.
Signal pest and attack with everything is okay... but its only 2 infect a turn and at most 3 damage, so its a very slow clock.
Post Sideboard you also need to judge your hands through the lens of "can this beat whatever artifact hate they brought in?" If your only Haymaker is an Overseer, then you have to worry about artifact destruction, creature removal, board wipes, and
Stony silence (all or which are ikely to come in against you), whereas if your Haymaker is
Cranial Plating or
Arcbound Ravager, it becomes a bit mre resilient to that sort of hate.
As for what your friends said, I agree, affinity ALWAYS wants to be the fastest deck, so early board presence is high priority. You more or less live by the mantra, "the best defence is a good offence", since you always want your opponent to focus on dealing with what you have over developing thier own board state.
Anywho, thanks for reading my novel of a comment. I hope you find it helpful. And if you have more questions, feel free to ask!