A fixie generally does not have a mechanism to let you coast. The pedals go around when the wheel goes around. You can even pedal backwards and make the bike go backwards. I have never ridden a fixie... got to be interesting!
Then there are singlespeed bikes. Those you can coast with. But if you have a seven speed derailleur on the bike now, that seven speed Huffy wheel will probably work and if it takes a 35 mm tire you can almost certainly push it a bit wider.
If you went single speed... the sprocket would depend on the chainring size too. Actually if you have a front derailleur then you pretty much need the rear derailleur too. The rear derailleur has the mechanism to take up the varying length of chain that happens when you switch between larger or smaller chainrings or sprockets.
One fun game I have heard people play - you can have a "flip flop" hub, with sprockets on both sides. If the sprockets differ in size by the same number of teeth as your chainrings, then you can switch the chain to the larger chainring up front at the same time you switch to the smaller sprocket in back. You could have a two speed fixie that way, without fussing with a varying chain length!
Endless fun!