Im replacing flooring in basement and main floor with same flooring.

If you want a good flow from room to room using the same/similar flooring throughout gives an open feel. Wood floors should match or be within the the same color family. Separate rooms like a basement can have different flooring. Baseboards should be the same as trim around doors and windows.

It is very tricky to get an exact match when doing a wood to wood transition. I have noticed doing a nice transition between the rooms, such as a row of brick tile makes the transition flow better.

Here is an example!

It will look best if the baseboards are the same color as the flooring

Not necessarily, if it looks good and your happy, leave it.

I would say keep it the same color it looks more uniformed

If you want an uniform look then you should keep the colors the same but if would like a different look in the basement from the other part of the house then you could go with a shade darker or lighter but same type flooring

I think a more uniform look makes your home flow better and in my opinion look much nicer. Good luck and stay safe!

No but you should check for squeaks and screw in better

Ah, the rest of your question is here. It'll look better if everything matches but if that's not in the budget, it's not the end of the world. Just get as close as you can.

No you do not need to change the floor boards just re enforce them so they don’t squeak.

It all depends on the look you want. Flooring in 2 levels of the house don't need to be the same unless it continues down the stairs. That would be a concern if everything was carpet.