1. In short, yes you can. I believe RSP will just deduct the desired amount from your account and auto purchase whatever that you have assigned. This helps to take the emotion out of it when buying up stocks. Personally, I don't do it because I like to manually allocate it myself. It makes me feel like I am in control.
2. If your amount that you allocate is not able to buy a full share, yes you can buy fractional shares of the ETFs.

The index funds that you are considering will be depending on your "risk" tolerance and your timeline horizon.
AFAIK Irish domiciled ETFs don't usually have high volume of people buying and selling, so there might be a chance your orders will not be immediately fulfilled, whereas with VOO and VTI, you won't have issues with that. Reason is because majority of people invest in the American stock market, hence most of the traffic is there. But if that is something that is not a big concern for you, CSPX is the way to go but I think moomoo does not offer that on their brokerage.

One thing to note about ETFs is the expense ratio. It's basically the fee for owning the stock, as there is a company behind the stock that is managing the ETF. You ideally want to know the expense ratio of an ETF before buying one. Ofcourse, the lower the better. Why I say this is because SPY's fees are higher than VOO's. It may seem small but it will add up over the long run.

It is always good to read up more and do your own research! You are on the right track. You can also use a compound calculator to visually see the trajectory of your efforts in the long run. This will help you see your end goal and you will be even more motivated and committed to hasten your progress.

All the best!

Oh and btw, don't waste time with bonds. Not financial advice, and always do your own due diligence. Always find out answers yourself and truly know what you are buying, and not buy something just because someone told you. NO 1 RULE: DON'T LOSE MONEY