Shimano make and excellent 8-speed hub which requires very little maintenance and IMO is ideal for a commuter bike. If the bike has horizontal rear dropouts, there's no need for a sprung tensioner. I also ran a 1x6 derailleur setup for a while and never experienced issues with the chain dropping off, even with a 'derailleur' crankset. Since a 7-speed freewheel/cassette is only marginally wider, it shouldn't be a problem. If you can fit bashguards either side of the chainring this should help as well. I would shorten the chain to increase the tension, since this can help keep it on and you don't need it as long with only a single chainring.