silicon over grout will not do any good. Remember the need for silicon is movement a bit. grout will not move and will only break. So, on any area where movement is likely do not use grout, use silicon. As to your question about a large gap, use a product called backer rods, come in three sizes and for filling in larger gaps so caulk can then be applied.
every forum board has moderator or moderators for the monitoring, and a visionary person to keep things working right. If a post was answered by moderator title that does not mean that a post answered by non moderator title is any less creditable. We have a host of very professional people on board from all walks of construction that jump in and assist.