The planting distance probably has more to do with your gardening experience level and the conditions under which they're grown. If you grow them in a 'garden' area that is essentially bare soil, it's a lot easier to keep their potential spread in check. If you go out into the yard, dig a hole and plant them, when they begin to spread you may not see the new shoots emerging due to them being camouflaged amongst the grass or other vegetation nearby. But yes, you can grow them closer than 5 feet. I should add that I'm not familiar with how Glacier grows but by looking at the typical yield I would assume that the sidearms would probably be about the same length as Cascade (about 2-3 feet). They can reach that distance toward your Fuggles which don't produce any appreciable sidearms for me so I don't think you would have to worry about them getting tangled.
Mulch is always a good idea to keep weeds down and conserve moisture. Also, the benefit of something like compost is that it will slowly help to nourish the plants.
Once the crowns become more mature they will be able to support more vines to produce a larger crop. I grow them on poles and by stringing 5-6 vines per pole can yield the equivalent of 3 or 4 plants off of one crown.
Keep in mind though, growing hops isn't like building a house or something. If you see that the distance is too close you can always just dig them up and move after the first year. Hope that helps.