If a rod does not have the plastic thumb piece and its 6-7 ft does that mean it is just for spinning reels? Or are the rods without the plastic thumb piece used for baitcaster reels also?

And, if two rods are the use the same ratio of line and lure weight and action, is the length of the rod (either 6 or 6.5 ft) just your own personal preference?

Im not really sure i understand your questions but ill try....Baitcasters do have a piece of "plastic" that comes out that your fingers go in between to help with grip - my thumb doesnt go anywhere near it -not sure why you call it a thumb piece. Spinning rods do not have that piece, the base of the spinning reel acts as that piece. The length of the rod has nothing to with whether it is spinning or casting.

Length comes down to preference, some prefer them shorter for better accuracy but sacrifice distance - hope that helps

Well not thumb,I didnt think before i typed that, but the little black piece at the bottom of the handle. Is what I am asking is does it matter if you have a baitcaster reel on a rod that does not that have that black piece on the handle? Meaning, without the black piece on the rod, does that make it just for spinning reels?

I'm not sure.

But I have noticed on spinning rods the line guides are slightly different. The first guide is bigger than the first on a baitcast rod.

All casting rods I own have a "thumb piece"

A spinning rod does not have what you are talking about if I understand what you are saying.

There are some bait casting rods that do use a trigger less reel seat, they are more suited for deep rod holders. The best way to tell the differences between a trigger less bait casting rod and a spinning rod are the guides and the fact the reel mounts under slung on spinning rods.

Tight Lines!!!! 

So baitcasters will cast better with the smaller guides than the larger guides?

And, it would be better to have a 6.5 ft rod instead of a 6 ft on a baitcaster reel?