Any reason why I would need a 7WT for catching River Bass and White Bass or will a 6WT do?

I anticipate catching smaller fish for the most part and fishing smaller flies.

The target area will be the Brazos River below and above Whitney.

I also plan on getting an 8WT rod as well do go after Lake Bass and Redfish.

Nope, no reason at all, unless you just really want a 7 weight. That's an unusual rod weight, sort of the 16 gauge of fly rods. It sits in a sweet spot that those who prefer, really enjoy. Others will defer to 6 or 8.

But if you're also headed for an eight weight, that is further reason to go with a six. Loads more lines available for sixes is another argument.

Yeah, it would be redundant to have both a 7WT and an 8WT. I would probably need to go with a 9WT instead if I went with the 7WT.

Also, if I ever head up to OK to go after those stocked trout, the 6WT would be a good match for those conditions and I could continue to use my 4WT with smaller flies. Part of this is also having a little extra power for throwing size 10 flies.

I have an old Flyrod that is a 7-wt , can't remember the name right off but the rod is 2-pc and actually is a very nice rod but it's also only 7-ft . Right now the reel on it is junk and the line is no good . Perhaps some day i'll buy a new cheap reel and line to play with it . My other rigs are both 5-wt . Nice rods !

The rod and line weight are more about what you are throwing and the wind conditions than what you are fishing for. If you aren't throwing heavy flies or fighting high winds you would be fine with a 2, 3, or 4 weight, and a 5 weight would probably handle any flies you'd ever throw in the river. If winds are a problem, then you'll likely want something heavier than a 5.

Looks like you have smaller flies covered. 4/5 is a all around trout rod so get the 8. You will be happy you did when throwing larger bass bugs (big coneheads etc.) And then if you just want to have another toy get a 6

I would go with the three or four personally but that's just me. All mentioned rods will do the job!

I have some size 10 flies so I'm trying to make sure I have a rod that can cast them in the wind.

A five or a six will be plenty of rod unless the wind is really fierce. A one or two weight will cast #10 flies easily (it just won't fight the wind well).