Ethics on shortening project in grant application to raise chances of success
Yes, of course this should not be a problem.  The funding agencies want to see projects that are on a realistic scale.  If it will probably not be possible for you to achieve your main goals, given the time and money allotted to the project, that certainly makes it seem like a poor investment.  Conversely, the funders know that if you reach your projected goals early, you are going to use the remainder of your funding to extend the research further.  So you should try to set achievable goals in your proposal, but if you manage to exceed them, nobody is going to be unhappy.

Naturally, cutting major tasks out of your proposal is going to require some rethinking of the justification.  If the projects you think do have time to complete are less interesting by themselves than the later projects you will need to cut, they you still need to explain why they are important stepping stones toward the ultimately most important work (even if you won't be getting to that work yet).  It can be subtle to get this right, but think you would be wise to trust your advisor's judgement about how much to include in your proposal.