The pointy kind of metal screws are called self tapping.... but you can nip off the point and it will still thread itself into the hole you drill for it. Coat the hole with an anti corrosion product tef-gel or lanicoat. (I prefer the tef gel). The aluminum is pretty thin and the self tapping design will work well. I've used both aluminum and SS rivets on various boat applications. The SS is the strongest but many are wary of the corrosion issue. I found that aluminum will minimize the issues created by dissimilar metals, but...... wear and tear from the elements can affect the Non anodized fastener. That said, if you go with rivets, find the best quality aluminum you can. Another reason for using aluminum is that they are much easier to remove. SS is pretty damn hard to drill out making it easy to damage the aluminum surface and distort the hole. I learned this the hard way.
Last comment, while the self tapping screw can be trimmed to minimize its intrusion inside the mast, you cannot do the same with the "pop" style rivets. They are going to protrude into the spar significantly, though not sharp like the screw. In most cases this won't be an issue... I just wanted to remind you. Okay.... good luck... have fun in your C27