They do not use any magical rainbow colours but non of them except for enemies like snakes and stuff looks realistic, it is just that. When the monsters where spoken about in a trailer they said they made them look like and gave them an environment where you would expect them to realistically live and while it is true that you can see enemies living in area x there is still no way to actually see a giant fireball because no matter the colour, the design automatically rules out the possibility. Before you mentioned it I have already very well acknowledged that they tried to make it look fitting the environment, thus also the colours and all. But you will not see me saying with the colour choice I can believe that floating fireballs come down some stairs or that flans and nagas wait in my sewers below. The monsters still do not look like they could be real for the very reason that their appearance is just over the top different from our animals in wildlife, no matter the colours. Hence only stuff like those who just look like some normal animal mutation really are imaginable to some extent. An appearance matching the environment does not really qualify for me to be enough to look truly realistic. As said, they "match their world" but that's really it. Not more, thus not enough.
And what even is the problem for enemies to appear in VII, just with a bit of a different colouring. We already have seen Dirge of Cerberus and Crisis Core bringing in old monsters PS2/PSP style. It looked "more realistic graphics-wise" and still they were included. We would have to skip half of the bosses and monsters. And that will not happen. What people also seem to forget: a) The trailers were about Midgar. Midgar is a dark and dirty place where only the neon light signs are truly colorful. The rest of the world looks different. And even then people should not complain too much, because b) the world of Final Fantasy actually even on PS1 is already shown to have been heavily influenced by being abused with the whole energy consumption thing and all. So I very much expect a lot of dark colours and dying places simply because Final Fantasy VII always was that. People just tend to forget that because of the 1, 2 cartoony elements that are included. Yes, they are there. They are not all of it though.
Being a staple does not change that they are included but they look as ridiculous and impossible to exist as walking houses or matroschka barrels with eyes or other stuff. And it is Final Fantasy VII, so why do you think your second sentence would be a problem for the remake? They have a bunch of people working for the monster monsters and their behaviour. Of course they look up the old monsters in the original. But I will surely not cry when I realize that 1 out of gazillion monsters is missing. I will say "damnit, that was in the original and is not there now" and then continue to play for another 1000 hours.
And I am sorry but stuff like the Hellhouse can easily be pulled off in the remake. Also with a very cool look. I believe a mechanical house to exist after being created by some organization 100% more than running skeletons in FFXV, etc. And we know that undeads are also in VII. They would have to reduce the monster palette to 10% and this will not happen. They are out enemies then.