Punishers are very expensive for a very weak gun. You're only going to hit 10 of those shots on average, and wound with 2/3rds of them if you're shooting anything T4, which most of your targets will be. That will end up being like 6-7 wounds on a Space Marine squad, which might kill 2. A Battlecannon pieplate will do significantly better. Heavy bolter sponsons will do some extra wounds if you're shooting infantry, as Leman Russes aren't really suited to tank hunting.
Heavy stubbers are awesome, but are often dropped for the sake of efficiency. A lot of net lists focus on minmaxing and building cookie cutter armies that all look and play the same. Heavy stubbers give 3 shots at strength 4 at 36" range, which is a healthy addition to any firing platform. I never leave home without em. If you've got the points to spare, they're useful.
Heavy weapons teams are awful for mobile anti tank. They need to stand still to shoot. If you leave them in a gunline in your deployment zone they'll do fine though. For mobile anti tank you'll want outflanking sentinels with autocannons or the trusty standby of veterans with melta guns in Chimeras/Valks. As for grav-chute insertion, I've only done it once and lost 3/4 of my squad. It was hilarious and ineffective. In theory it could work.
For penal legion, you could get Catachans and paint their clothes orange. That's the easiest way to go about it. You could also convert some, using Victoria Lamb's Penal Guard conversion kits:
http://www.victorialamb.com/store.html they're about a third of the way down the page.