I have finally chosen for my first load:

(.45colts in a Taurus .454)
Hornady .45colt 300gr .452dia XTP MAG,
Starline brass with Hornady H110 for my higher pressure loads and Alliant Unique for my lower presure loads.
I have available to me Winchester large pistol primers.

Will these work or would CCI or Colt primers be better?

Can I get away with just .300 MAG XTP for both my high and low pressure loads or will I also need something smaller like 250gr to keep accuracy consistent?

Is .452 diameter ok for .45colt in a .454 gun or do I need .451 diameter?

I read you should use a Lee taper crimp die for the plus pressure .454 loads.
Should I also use it for the plus pressure .45 loads?

I was going to use a #20 RCBS shellplate with the Lee taper die in my RCBS press but I was told the RCBS shellplate isnt compatable with the Lee die but the Lee die is compatable with the RCBS press... now im confused (sigh)
Please tell me what goes with what.
I already have the RCBS press.

Once again thank you for your time!
This is exhausting but I am figuring it out.

You are Ok with WLP .452" Most revolver use a roll crimp. Don't know about the 300 or 250 gr. or the shell plate?

Your components look fine to me. I run both of these powders in my 45 colt with no trouble. Good choice of brass.

Primer type doesn't matter, as long as you are consistent. I have used winchester primers, and they work fine. I did settle on CCI primers for everything at this point - but that's just my preference.

Use a roll crimp for all loads in your revolver. I would advise performing the bullet seating and crimping as separate operations, especially since you are new to this. I still do them separately, but this is because I have a Lee factory crimp die available. It's easy to set the dies wrong and overcrimp so the rounds will not chamber. If you seat the bullet in a separate step, you can worry about adjusting the crimp to be just right without thinking about seating the bullet at the same time.

Good choice of bullet. The 300 gr XTP works very well for me at +P loads as well as "reduced" or "standard" 45 Colt loads. The XTP Mag bullet is tougher, as it was made for the faster speeds of the 454, and it likely will not expand as well at slower speeds in your reduced loads. But hey, the bullet is already .452" - that's a big hole even without expanding.

Accuracy is a big unknown. All you can do is try these components in your particular gun and see what it likes. As a rule, you don't need to go to a lower bullet weight for slower loads. The only concrete advantage would be a higher velocity with the 250 gr bullets as compared to the 300 gr bullets at the same chamber pressure level.