With a 401k, can I sign up first, and then switch to different investment types and buy/sell them later?

For example, our company's 401K has an easy choice of "BlackRock US index fund" (for Dow Jones 30 DJI) using Fidelity, and does not require opening a broker account. Should I just start using the 401K account ASAP so I don't miss any paycheck contribution and company matching?

And then, after one or two months, we can open up a brokerage account, buy Fidelity 500 (FXAIX) or SPY, and then in our account, we will have 2 investment types? And then after 2 or 5 years, we can see how they perform, and we can sell one and consolidate into the other one? I think one advantage of it being in 401K is that even if there is any gain, we don't have to pay tax on it for this switch of investment type?