Would you recommend venetian plaster on walls to this tiled powder room? Colour?
And what kind of light n mirror shape can I add to complete the look. I have a Devonshire pedestal on the left and a small vanity table on right.
I like French/ Italian look.

Re bathroom styles. I would do only two different styles. 3 would be too many and would ruin the flow of the house (country style - eliminate).

mmmadrid, I think it is fine. It is a good idea to have some sort of unifying design element throughout to tie everything together. Like oiled bronze hardware in all 3 bathrooms, or subway tiled tub/showers. But a range of vanity styles is ok in my book. Kitchen Bath Collection carries affordable, quality vanities in all these styles. See link below:
http://kspoll.hubpages.com/hub/kitchen-bath-collection-bathroom-vanities

Are you doing custom or off the shelf. If doing custom here is a tip I have used a couple of times. Cupboard doors - instead of having just 2 doors with a huge space that only has one shelf area - what I did was to put a drawer in the bottom and 2 smaller doors above. Makes better use of the space.

@Angel18432 We are doing custom. @kspollock Let me know if you think this is enough to tie them together- Jack & Jill and Upstaitrs guest will have the same tile & ORB fixtures, the tubs are actually both inserts- wanted an insert for the kids & since the guest is an upstairs did an insert in there as well. Also all vanities will be clear alder but possibly different stains, except jill. jill might be painted white. Master is kind of its own thing- going a little more modern in there. Travertine, satin nickel, rock floor.

Oh and same granite in all the bathrooms too!

I think bathrooms can be quite different from each other in private spaces that attach to bedrooms as these are usually done to go with the bedroom I agree with not having too many different styles tough. I love vanities with all drawers like the Ikea Godmorgon, if your cabinet maker is not familiar , that is absolutely fabulous for storage and sometimes you get the best pricing for granite when you can do all counters the same but the other side of that is using remnants is a big money saver. I too would lose the country style and do the guest similar to the master .

Sounds like you have lots of similar finishes so as not to make the rooms too choppy.