Any carpets fitted with an underlay should be fitted with griper Road around the outsides of the room. The gap you will need to use will vary depending on the thickness of your carpet. I've been fitting for 18 years and on average I use about a 4mm gap around all outer edges to allow for the carpet to tuck into. Underlay should always be cut up to the inside edge of the gripper rods. It is correct that felt backed carpets can be fitted without using gripper rods straight to the floor with adhesive, the same as the old foam backed carpets used to. You may find if you get a fitter that knows what he's doing they will be able to fit the underlay tight to the walls only allowing the same tucking gap you would allow using gripper rods and sticking the carpet to the underlay the same way you would if you were sticking it to the floorboards, stretching and tucking it the same way as if it was fitted on gripper rods, however this method, although reasonably effective, is not the correct way to do it. When I say sticking, I mean using a spray adhesive designed for textile products. The underlay, providing its being fitted in a domestic area, should be fixed using either spray adhesive or staples dependant on whether the sub floor is concrete or wood. Underlay would only need to be fully glued to the floor if the carpets being fitted are either of the natural variety, which would also need to be glued to the underlay (durafitting) using other much stronger glues, or any carpets that are being fitted in a commercial setting, areas of particular high traffic, or areas where people may be using walking aids or wheelchairs. The underlay used in those settings will be specific for those kind of commercial settings anyway.
Hessian/action backed carpets - need gripper and underlay
Felt backed carpets - can be fitted straight to the floor using spray adhesive as a gripping agent, or fitted on underlay with gripper rods
For any gaps in the floor unfortunately unless boarded out, you will literally be papering over the cracks and any imperfection over time will start to show through. Much quicker when fitted without an underlay. If it is only gaps and not un even, I would recommend using some hardboard/eucalyptus board to hide any discrepancies, however if the floor is particularly bad with gaps and some uneven boards. Some 6mm plywood may be the better option