Mainstream coins or altcoins. I mean it's bitcoin and then everything else is altcoins so I think you need to figure out what you are asking first. But if you are talking large cap vs small cap coins it all depends on your goal and appetite for risk. There is not one size fits all answer here.
Yeah bitcoin is the only coin that needs to be bought. Not sure what OP meant by mainstream coin but both popularly known newly launched coins, memecoins and shitcoins are not worth the time nor money. Bitcoin is mainstream enough in my opinion, heck it is the only one that is globally recognised as a notorious new currency.
Large cap always over small cap. You can never get a good long term output from small cap coins with small factions controlling the demand supply and why bother to crack your head over it anyway when you have bitcoin at hand.