Gravel can be used as a decorative mulch - you can use horticultural grit which can deter (but not guarantee against) slugs and snails or 10mm or 20mm gravel (homebase is nice natural colours and good quality - not too much dust/sand). Wash gravel to remove sandy bits before using.
Use a good thick layer to keep in moisture and cut out light to prevent weed seeds from setting. Gravel/grit also stops plants getting splashed with mud in heavy rain or when watering using a hosepipe.

Or you can go with the coloured grits - alpine gold etc if you want something brighter.

Yes - and no. The trouble with topping off pots with something is, its easy to not realise they're dry and need watering, because you can't see or feel the compost.

Have good drainage in the pots with plenty of grit incorporated in the compost and water every day.....pots dry out very quickly. With good drainage there is no fear in over watering.